Buck: Import all bucklets in bucklet.defs

This means all bucklets used by Gitiles are loaded on every buck
invocation.

Arguably it's a performance hit if you try to build a target that
doesn't require the bucklets, but for a small project like Gitiles where
every rule requires loading at least one bucklet, the cost should be
minimal.

Inspired-by: Dave Borowitz <dborowitz@google.com>
Change-Id: Ic1c8048437876b5bfd7cd503b32bd97649e4402c
10 files changed