Use one mutator for all bp2build conversion.

Each conversion required defining a separate mutator, which will each
operate on _all_ modules and requires each to repeat checks whether the
mutator should operator. Instead, we introduce a single mutator and
modules can define a ConvertWithBp2build to implement bp2build
conversion for that module.

Test: bp2build.sh
Bug: 183079158
Change-Id: I99d4b51f441c2903879092c5b56313d606d4338d

bp2build/genrule_conversion_test.go

@@ -28,7 +28,6 @@ func runGenruleTestCase(t *testing.T, tc bp2buildTestCase) {
 	t.Helper()
 	(&tc).moduleTypeUnderTest = "genrule"
 	(&tc).moduleTypeUnderTestFactory = genrule.GenRuleFactory
-	(&tc).moduleTypeUnderTestBp2BuildMutator = genrule.GenruleBp2Build
 	runBp2BuildTestCase(t, registerGenruleModuleTypes, tc)
 }