Handle already existing targets of different name

In other words, if, in bp2build, module "foo" would generate "foo",
and "foo_two", and "foo_two" already exists in a build file,
bp2build should label "foo" as being unconvertible.

Fixes: 301321658
Fixes: 301312582
Bug: 285631638
Test: Unit tests
Test: Verified that `m bp2build` results in bit-for-bit identical
contents for out/soong/bp2build before and after this change.

Change-Id: Icbbdd69fce83579ec9b172d04b2bf1f294698f70

bp2build/java_binary_host_conversion_test.go

@@ -114,7 +114,7 @@ func TestJavaBinaryHostLibs(t *testing.T) {
 	runJavaBinaryHostTestCase(t, Bp2buildTestCase{
 		Description:             "java_binary_host with srcs, libs.",
 		Filesystem:              testFs,
-		StubbedBuildDefinitions: []string{"prebuilt_java-lib-dep-1"},
+		StubbedBuildDefinitions: []string{"java-lib-dep-1", "java-lib-dep-1-neverlink"},
 		Blueprint: `java_binary_host {
     name: "java-binary-host-libs",
     libs: ["java-lib-dep-1"],