Add explicitlyImpl to shared library dependency tags

Instead of testing that explicit implementation libraries are not
incorrectly used in an apex after the fact add the explicitlyImpl
flag to dependency tags and use it in cc.DepIsInSameApex and
rust.DepIsInSameApex to require that explicit implementation
dependencies are in the same apex.

Bug: 372543712
Test: go test ./...
Change-Id: Ia968e06b578d5ab886a965c3981565d4895dddcd