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()`.

runtime/lua/vim/pack.lua

@@ -70,19 +70,15 @@
 ---      To discard updates execute |:quit|.
 ---    - (Optionally) |:restart| to start using code from updated plugins.
 ---
----Switch plugin's version:
----- Update 'init.lua' for plugin to have desired `version`. Let's say, plugin
----named 'plugin1' has changed to `vim.version.range('*')`.
----- |:restart|. The plugin's actual revision on disk is not yet changed.
----  Only plugin's `version` in |vim.pack-lockfile| is updated.
----- Execute `vim.pack.update({ 'plugin1' })`.
+---Switch plugin's version and/or source:
+---- Update 'init.lua' for plugin to have desired `version` and/or `src`.
+---  Let's say, the switch is for plugin named 'plugin1'.
+---- |:restart|. The plugin's state on disk (revision and/or tracked source)
+---  is not yet changed. Only plugin's `version` in |vim.pack-lockfile| is updated.
+---- Execute `vim.pack.update({ 'plugin1' })`. The plugin's source is updated.
 ---- Review changes and either confirm or discard them. If discarded, revert
----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.
+---  `version` change in 'init.lua' as well or you will be prompted again next time
+---  you run |vim.pack.update()|.
 ---
 ---Freeze plugin from being updated:
 ---- Update 'init.lua' for plugin to have `version` set to current revision.
@@ -922,12 +918,6 @@ function M.add(specs, opts)
   local plugs_to_install = {} --- @type vim.pack.Plug[]
   local needs_lock_write = false
   for _, p in ipairs(plugs) do
-    -- 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
@@ -1172,10 +1162,18 @@ function M.update(names, opts)
 
   -- Perform update
   local timestamp = get_timestamp()
+  local needs_lock_write = opts.force --- @type boolean
 
   --- @async
   --- @param p vim.pack.Plug
   local function do_update(p)
+    -- Ensure proper `origin` if needed
+    if plugin_lock.plugins[p.spec.name].src ~= p.spec.src then
+      git_cmd({ 'remote', 'set-url', 'origin', p.spec.src }, p.path)
+      plugin_lock.plugins[p.spec.name].src = p.spec.src
+      needs_lock_write = true
+    end
+
     -- Fetch
     if not opts._offline then
       -- Using '--tags --force' means conflicting tags will be synced with remote
@@ -1197,8 +1195,11 @@ function M.update(names, opts)
     or 'Downloading updates'
   run_list(plug_list, do_update, progress_title)
 
-  if opts.force then
+  if needs_lock_write then
     lock_write()
+  end
+
+  if opts.force then
     feedback_log(plug_list)
     return
   end