sync: Skip copyfile/linkfile for unavailable projects

Commit 5534f16 made UpdateCopyLinkfileList to retry _CopyAndLinkFiles
for all projects in the manifest to handle directory-to-symlink
transistions. However, for partial sync's (e.g. repo sync <project>),
projects not included in the sync may not have their local directories
populated, attempts to perform the operation on unavailable projects
would result in dangling symlinks, or errors like:

error: Cannot copy file <source> to <destination>

Fix this by adding a check inside _CopyAndLinkFiles in project.py to
so these operations happen only when the project's worktree exists
and is a directory.

TAG=agy
CONV=90f5fca7-2199-4017-8f62-a010b0ab1dbf

Change-Id: I9ae1e8d62d8fd129cb4535e574c38339c87af441
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/587501
Reviewed-by: Mike Frysinger <vapier@google.com>
Tested-by: Xin Li <delphij@google.com>

project.py

@@ -1575,6 +1575,8 @@ class Project:
         self._InitHooks()
 
     def _CopyAndLinkFiles(self):
+        if not self.worktree or not platform_utils.isdir(self.worktree):
+            return
         for copyfile in self.copyfiles:
             copyfile._Copy()
         for linkfile in self.linkfiles: