feat(pack)!: change src of installed plugin inside update() in place

Problem: Changing `src` of already installed plugin currently takes
  effect immediately inside `vim.pack.add()` and acts as "delete and
  later fresh install". Although more robust, this might lead to
  unintentional data loss (since plugin is deleted) if the plugin was
  manually modified or the new source is not valid.
  Also this introduces unnecessary differentiation between "change
  `version`" and "change `src`" of already installed plugin.

Solution: Require an explicit `vim.pack.update()` to change plugin's
  source. It is done by conditionally changing `origin` remote of the
  Git repo. The effect does not require update confirmation in order to
  have new changes fetched from the new `src` right away.

  If in the future there are more types of plugins supported (i.e. not
  only Git repos), also do extra work (like delete + install) during
  `vim.pack.update()`.

test/functional/plugin/pack_spec.lua

@@ -790,46 +790,6 @@ describe('vim.pack', function()
       eq(false, pack_exists('basic'))
     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()
@@ -1142,7 +1102,7 @@ describe('vim.pack', function()
 
   describe('update()', function()
     -- Lua source code for the tested plugin named "fetch"
-    local fetch_lua_file = vim.fs.joinpath(pack_get_plug_path('fetch'), 'lua', 'fetch.lua')
+    local fetch_lua_file
     -- Tables with hashes used to test confirmation buffer and log content
     local hashes --- @type table<string,string>
     local short_hashes --- @type table<string,string>
@@ -1157,6 +1117,7 @@ describe('vim.pack', function()
       repo_write_file('fetch', 'lua/fetch.lua', 'return "fetch main"')
       git_add_commit('Commit from `main` to be removed', 'fetch')
 
+      fetch_lua_file = vim.fs.joinpath(pack_get_plug_path('fetch'), 'lua', 'fetch.lua')
       hashes = { fetch_head = git_get_hash('HEAD', 'fetch') }
       short_hashes = { fetch_head = git_get_short_hash('HEAD', 'fetch') }
 
@@ -1726,6 +1687,52 @@ describe('vim.pack', function()
       eq(hashes.fetch_new, get_lock_tbl().plugins.fetch.rev)
     end)
 
+    it('can change `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)
+
+      local function assert_origin(ref)
+        -- Should be in sync both on disk and in lockfile
+        local opts = { cwd = pack_get_plug_path('basic') }
+        local real_origin = system_sync({ 'git', 'remote', 'get-url', 'origin' }, opts)
+        eq(ref, vim.trim(real_origin.stdout))
+
+        eq(ref, get_lock_tbl().plugins.basic.src)
+      end
+
+      n.clear()
+      watch_events({ 'PackChangedPre', 'PackChanged' })
+
+      assert_origin(basic_src)
+      exec_lua(function()
+        vim.pack.add({ { src = defbranch_src, name = 'basic' } })
+      end)
+      -- Should not yet (after `add()`) affect plugin source
+      assert_origin(basic_src)
+
+      -- Should update source immediately (to work if updates are discarded)
+      exec_lua(function()
+        vim.pack.update({ 'basic' })
+      end)
+      assert_origin(defbranch_src)
+
+      -- Should not revert source change even if update is discarded
+      n.exec('quit')
+      assert_origin(defbranch_src)
+      eq({}, exec_lua('return _G.event_log'))
+
+      -- Should work with forced update
+      n.clear()
+      exec_lua(function()
+        vim.pack.add({ basic_src })
+        vim.pack.update({ 'basic' }, { force = true })
+      end)
+      assert_origin(basic_src)
+    end)
+
     it('shows progress report', function()
       track_nvim_echo()
       exec_lua(function()