bp2build: Expand check if filegroup contains proto

Previously we looked for proto as suffix, however, some filegroups use
"<foo>-proto-srcs" or "<bar>-proto-sources", instead we look for proto
as a distinct word in a filegroup name.

Test: go test soong tests
Change-Id: Icf916a84304a02617efff9768e5b82d5ffe658bd