Fix watching paths in undefined repos in repo rules

Watching a path in an undefined repo (eg. `$output_base/external/i_am_undefined/blah`) currently causes an NPE because we expect it to need a skyframe restart when it actually doesn't. The underlying reason is rather convoluted, but the gist is that we request `RepositoryDirectoryValue.Key(@@i_am_undefined)` explicitly, when we don't actually have to -- we can simply use `FileValue.Key($output_base/external/i_am_undefined/blah` directly, and rely on the fact that `FileFunction` knows to request the corresponding `RepositoryDirectoryValue` and behave appropriately when the repo is undefined. (This is due to logic in `ExternalFilesHelper.maybeHandleExternalFile`.)

Additionally, this PR removes the `rctx.symlink(watch_target=)` parameter. The symlink target's existence and its contents should not affect the creation of the symlink at all, so there's no reason to ever watch it.

Fixes #21483.