Create zip_file() rule

This doesn't work quite the same way as Fileset(). But it should be able
to serve as a decent replacement.

The important part of this design is that zips can depend on other zips.
Therefore definitions don't have to be monolithic. This will be
important when migrating the Domain Registry codebase, because our
Fileset() definitions are dispersed across many BUILD files. So we'll be
able to migrate to zip_file() with the least amount of intrusiveness.

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=129034885

javatests/google/registry/builddefs/override/hello.txt

@@ -0,0 +1 @@
+OMG IM AN OVERRIDE