fix(plugins): fix failure on repeated installation

When a plugin or flavor is first installed and later updated, the
link to the plugin or flavor's directory is already in place. This
caused an error when the plugin or flavor was installed again (updated).

Now, reinstalling checks if the symlink is already in place and does
nothing if it is.

lua/yazi/plugin.lua

@@ -63,11 +63,28 @@ end
 ---@param to string
 ---@return YaziSpecInstallationResultSuccess | YaziSpecInstallationResultFailure
 function M.symlink(spec, to)
+  -- Check if the symlink already exists, which will happen on repeated calls
+  local existing_stat = vim.uv.fs_lstat(to)
+  if
+    existing_stat
+    and existing_stat.type == 'link'
+    and vim.uv.fs_readlink(to) == spec.dir
+  then
+    ---@type YaziSpecInstallationResultSuccess
+    local result = {
+      message = 'yazi.nvim: already installed ' .. spec.name,
+      from = spec.dir,
+      to = to,
+    }
+    -- don't notify about this as it's a common case
+    return result
+  end
+
   local dir = vim.uv.fs_stat(spec.dir)
   if dir == nil or dir.type ~= 'directory' then
     ---@type YaziSpecInstallationResultFailure
     local result = {
-      error = 'yazi plugin/flavor directory does not exist',
+      error = 'source directory does not exist',
       from = spec.dir,
       message = 'yazi.nvim: failed to install',
     }

tests/yazi/plugin_spec.lua

@@ -47,10 +47,7 @@ describe('installing a plugin', function()
         name = 'test-plugin-2',
       }, { yazi_dir = yazi_dir })
 
-      assert.is_equal(
-        result.error,
-        'yazi plugin/flavor directory does not exist'
-      )
+      assert.is_equal(result.error, 'source directory does not exist')
       assert.is_equal(result.from, plugin_dir)
     end)
   end)
@@ -74,4 +71,19 @@ describe('installing a plugin', function()
       assert.are.same(flavor_dir, symlink)
     end)
   end)
+
+  describe('symlink', function()
+    it("doesn't complain if the symlink already exists", function()
+      local source = vim.fs.joinpath(base_dir, 'source-dir')
+      local target = vim.fs.joinpath(base_dir, 'target-dir')
+
+      vim.fn.mkdir(source)
+
+      local result = plugin.symlink({ name = 'source', dir = source }, target)
+      assert.are.same(result.error, nil)
+
+      local result2 = plugin.symlink({ name = 'source', dir = source }, target)
+      assert.are.same(result2.error, nil)
+    end)
+  end)
 end)