bp2build support for cc_prebuilt_library

while cc_prebuilt_library_(shared|static) were already supported,
the raw cc_prebuilt_library module type was not supported by
bp2build. This CL provides that functionality.

Also did a little bit of refactoring and cleaning.

Bug: 225938765
Test: Confirm output BUILD.bazel files look as expected. Confirm
      that build succeeds.
Change-Id: Ic004f88febc0e5b527f2ef420dbd96ecc55d2e09

bp2build/testing.go

@@ -84,8 +84,9 @@ type bp2buildTestCase struct {
 	expectedBazelTargets       []string
 	filesystem                 map[string]string
 	dir                        string
-	expectedErr                error
-	unconvertedDepsMode        unconvertedDepsMode
+	// An error with a string contained within the string of the expected error
+	expectedErr         error
+	unconvertedDepsMode unconvertedDepsMode
 }
 
 func runBp2BuildTestCase(t *testing.T, registerModuleTypes func(ctx android.RegistrationContext), tc bp2buildTestCase) {