Revert "Remove exception for relative CIPD"

This reverts commit 2d29261e.

Reason for revert: Both jbudorick and I asked you not to land this today :(.

Original change's description:
> Remove exception for relative CIPD
> 
> This seems to no longer be necessary, as relative paths 'just work' for
> CIPD deps now.
> 
> Change-Id: I3843548dc74d29af90e0c402f0459dbe60934423
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/1529132
> Reviewed-by: Dirk Pranke <dpranke@chromium.org>
> Reviewed-by: John Budorick <jbudorick@chromium.org>
> Commit-Queue: Brandon Tolsch <btolsch@chromium.org>

TBR=dpranke@chromium.org,jbudorick@chromium.org,btolsch@chromium.org

Change-Id: I1d9a771ef34dae0dec15499d7cf28e4ba5daf2ff
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/1531193Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Commit-Queue: Dirk Pranke <dpranke@chromium.org>

gclient.py

@@ -1878,6 +1878,10 @@ class CipdDependency(Dependency):
         should_recurse=False,
         relative=relative,
         condition=condition)
+    if relative:
+      # TODO(jbudorick): Implement relative if necessary.
+      raise gclient_utils.Error(
+          'Relative CIPD dependencies are not currently supported.')
     self._cipd_package = None
     self._cipd_root = cipd_root
     # CIPD wants /-separated paths, even on Windows.