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/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+)'