fix(pack): rename confirmation buffer to again use nvim-pack:// scheme

Problem: `nvim://` scheme feels more like a generalized interface that
  may be requested externally, and it acts like CLI args (roughly).
  This is how `vscode://` works.

  Anything that behaves like an "app" or a "protocol" deserves its own
  scheme. For such Nvim-owned things they will be called `nvim-xx://`.

Solution: Use `nvim-pack://confirm#<bufnr>` template for confirmation
  buffer name instead of `nvim://pack-confirm#<bufnr>`.

runtime/doc/lua-plugin.txt

@@ -283,7 +283,7 @@ mappings to a specific action by invoking `vim.lsp.buf.code_action()` with the
 
 Example: See `runtime/lua/vim/pack/_lsp.lua` for how vim.pack defines an
 in-process LSP server to provide interactive features in its
-`nvim://pack-confirm` buffer.
+`nvim-pack://confirm` buffer.
 
 ==============================================================================
 Troubleshooting                                   *lua-plugin-troubleshooting*

runtime/lua/vim/pack.lua

@@ -1057,7 +1057,7 @@ end
 local function show_confirm_buf(lines, on_finish)
   -- Show buffer in a separate tabpage
   local bufnr = api.nvim_create_buf(true, true)
-  api.nvim_buf_set_name(bufnr, 'nvim://pack-confirm#' .. bufnr)
+  api.nvim_buf_set_name(bufnr, 'nvim-pack://confirm#' .. bufnr)
   api.nvim_buf_set_lines(bufnr, 0, -1, false, lines)
   vim.cmd.sbuffer({ bufnr, mods = { tab = vim.fn.tabpagenr() } })
   local tab_id = api.nvim_get_current_tabpage()

runtime/lua/vim/pack/_lsp.lua

@@ -20,7 +20,7 @@ function methods.shutdown(_, callback)
 end
 
 local get_confirm_bufnr = function(uri)
-  return tonumber(uri:match('^nvim://pack%-confirm#(%d+)$'))
+  return tonumber(uri:match('^nvim%-pack://confirm#(%d+)$'))
 end
 
 local group_header_pattern = '^# (%S+)'