kleaf: Copy bazel_common_rules/exec here.

We are getting rid of bazel_common_rules soon, so copy necessary files
here. These rules don't quite work yet; it'll be fixed in follow up
changes.

Also rename BUILD to BUILD.bazel.

Bug: 329305827
Change-Id: Ie54ea10d9e96069a396902d74d3bbb2aa2bfc6ec
5 files changed