bp2build: don't generate filegroups (or error) if it contains a file

with the same name.

Also add capability to test for errors raised in bp2build mutators /
contexts.

This CL does two things to filegroups:

1) If the filegroup has only 1 source file with the same name as itself,
don't generate a filegroup target. Instead, dependents will depend
directly on the Bazel file target instead.

2) If the filegroup has more than 1 source file and 1 of them has the
same name as itself, the bp2build mutator will error out. If bp2build
on CI passes, it means that the source tree / product we're testing
against does not have such a case (which seems to be true for most
source trees).

Either way, this will allow us to be unblocked for most of the errant
filegroups (case 1) in the tree.

Test: CI
Test: New test cases in filegroup_conversion_test.go

Fixes: 194762573
Change-Id: I830c53efc8808569afe3c5f9f08436855bcdafed