Replace sync.Mutex with Sync.Map

createProtoLibraryTargetsForIncludeDirs uses a lock-protected plain map
to see if a directory has been handled previously during concurrent visits
of modules in bp2build.

From golang's docs https://pkg.go.dev/sync#Map, replacing sync.Mutex
with sync.Map will be faster in this use case since the keys (directores
here) are written just once but read many times.

Test: m bp2build
Change-Id: Ia2a471b4db5d2890fa6048bc05a17cebc5f686af