envsetup: add bmod to get Bazel label of a Soong module.

This benefits interactive workflows of folks who are used to typing just
module names on the command line, and lowers the cognitive load to find
the package name, and can be used to check if a module is converted.

Can be used as :

$ bmod libaapt2
//frameworks/base/tools/aapt2:libaapt2
$ b build $(bmod aapt2)

If the mod is not in the bp2build metrics file, it returns:

$ bmod nonexistent
nonexistent is not converted to Bazel.

Test: build/make/core/tests/b_tests.sh
Change-Id: I245665a393621a47598e6743c0fedcd87ac33631