Modify Bazel exporter's findRule to only return Rule ptr.

Have findRule() only return a build.Rule pointer and not an error.
If no rule is found nil will be returned. The caller will determine
whether this is an error.

Bug: skia:563656
Change-Id: I5ff0bf4465ff12273716224fbb899a334b98c997
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/566879
Reviewed-by: Kevin Lubick <kjlubick@google.com>
Commit-Queue: Chris Mumford <cmumford@google.com>
5 files changed