feat(pack): update add() to handle source change for installed plugin

Problem: Changing `src` of an existing plugin cleanly requires manual
  `vim.pack.del()` prior to executing `vim.pack.add()` with a new `src`.

Solution: Autodetect `src` change for an existing plugin (by comparing
  against lockfile data). If different - properly delete immediately and
  treat this as new plugin installation.

  Alternative solution might be to update `origin` remote in the
  installed plugin after calling `vim.pack.update()`. Although, doable,
  this 1) requires more code; and 2) works only for Git plugins (which
  might be not the only type of plugins in the future). Automatic
  "delete and clean install" feels more robust.

runtime/doc/pack.txt

@@ -280,6 +280,10 @@ Switch plugin's version:
   changes in 'init.lua' as well or you will be prompted again next time you
   run |vim.pack.update()|.
 
+Switch plugin's source:
+• Update 'init.lua' for plugin to have desired `src`.
+• |:restart|. This will cleanly reinstall plugin from the new source.
+
 Freeze plugin from being updated:
 • Update 'init.lua' for plugin to have `version` set to current revision. Get
   it from |vim.pack-lockfile| (plugin's field `rev`; looks like `abc12345`).
@@ -354,10 +358,12 @@ add({specs}, {opts})                                          *vim.pack.add()*
     Add plugin to current session
     • For each specification check that plugin exists on disk in
       |vim.pack-directory|:
-      • If exists, do nothing in this step.
+      • If exists, check if its `src` is the same as input. If not - delete
+        immediately to clean install from the new source. Otherwise do
+        nothing.
       • If doesn't exist, install it by downloading from `src` into `name`
-        subdirectory (via `git clone`) and update revision to follow `version`
-        (via `git checkout`).
+        subdirectory (via partial blobless `git clone`) and update revision to
+        match `version` (via `git checkout`).
     • For each plugin execute |:packadd| (or customizable `load` function)
       making it reachable by Nvim.
 

runtime/lua/vim/pack.lua

@@ -77,6 +77,10 @@
 ---any changes in 'init.lua' as well or you will be prompted again next time
 ---you run |vim.pack.update()|.
 ---
+---Switch plugin's source:
+---- Update 'init.lua' for plugin to have desired `src`.
+---- |:restart|. This will cleanly reinstall plugin from the new source.
+---
 ---Freeze plugin from being updated:
 ---- Update 'init.lua' for plugin to have `version` set to current revision.
 ---Get it from |vim.pack-lockfile| (plugin's field `rev`; looks like `abc12345`).
@@ -769,9 +773,11 @@ end
 --- Add plugin to current session
 ---
 --- - For each specification check that plugin exists on disk in |vim.pack-directory|:
----     - If exists, do nothing in this step.
+---     - If exists, check if its `src` is the same as input. If not - delete
+---       immediately to clean install from the new source. Otherwise do nothing.
 ---     - If doesn't exist, install it by downloading from `src` into `name`
----       subdirectory (via `git clone`) and update revision to follow `version` (via `git checkout`).
+---       subdirectory (via partial blobless `git clone`) and update revision
+---       to match `version` (via `git checkout`).
 --- - For each plugin execute |:packadd| (or customizable `load` function) making
 ---   it reachable by Nvim.
 ---
@@ -804,13 +810,19 @@ function M.add(specs, opts)
   local plugs_to_install = {} --- @type vim.pack.Plug[]
   local needs_lock_write = false
   for _, p in ipairs(plugs) do
-    -- TODO(echasnovski): check that lock's `src` is the same as in spec.
-    -- If not - cleanly reclone (delete directory and mark as not installed).
+    -- Allow to cleanly change `src` of an already installed plugin
+    if p.info.installed and plugin_lock.plugins[p.spec.name].src ~= p.spec.src then
+      M.del({ p.spec.name })
+      p.info.installed = false
+    end
+
+    -- Detect `version` change
     local p_lock = plugin_lock.plugins[p.spec.name] or {}
     needs_lock_write = needs_lock_write or p_lock.version ~= p.spec.version
     p_lock.version = p.spec.version
     plugin_lock.plugins[p.spec.name] = p_lock
 
+    -- Register for install
     if not p.info.installed then
       plugs_to_install[#plugs_to_install + 1] = p
       needs_lock_write = true

test/functional/plugin/pack_spec.lua

@@ -611,6 +611,46 @@ describe('vim.pack', function()
       eq({ { '.git', 'directory' } }, entries)
     end)
 
+    it('allows changing `src` of installed plugin', function()
+      local basic_src = repos_src.basic
+      local defbranch_src = repos_src.defbranch
+      exec_lua(function()
+        vim.pack.add({ basic_src })
+      end)
+      eq('basic main', exec_lua('return require("basic")'))
+
+      n.clear()
+      watch_events({ 'PackChangedPre', 'PackChanged' })
+      exec_lua(function()
+        vim.pack.add({ { src = defbranch_src, name = 'basic' } })
+      end)
+      eq('defbranch dev', exec_lua('return require("defbranch")'))
+
+      -- Should first properly delete and then cleanly install
+      local log_simple = vim.tbl_map(function(x) --- @param x table
+        return { x.event, x.data.kind, x.data.spec }
+      end, exec_lua('return _G.event_log'))
+
+      local ref_log_simple = {
+        { 'PackChangedPre', 'delete', { name = 'basic', src = basic_src } },
+        { 'PackChanged', 'delete', { name = 'basic', src = basic_src } },
+        { 'PackChangedPre', 'install', { name = 'basic', src = defbranch_src } },
+        { 'PackChanged', 'install', { name = 'basic', src = defbranch_src } },
+      }
+      eq(ref_log_simple, log_simple)
+
+      local ref_messages = table.concat({
+        "vim.pack: Removed plugin 'basic'",
+        'vim.pack: Installing plugins (0/1)',
+        'vim.pack: 100% Installing plugins (1/1)',
+      }, '\n')
+      eq(ref_messages, n.exec_capture('messages'))
+
+      local defbranch_rev = git_get_hash('dev', 'defbranch')
+      local ref_lock_tbl = { plugins = { basic = { rev = defbranch_rev, src = defbranch_src } } }
+      eq(ref_lock_tbl, get_lock_tbl())
+    end)
+
     it('can install from the Internet', function()
       t.skip(skip_integ, 'NVIM_TEST_INTEG not set: skipping network integration test')
       exec_lua(function()