Fix possible orphaned proto_library targets

This is a fix for aosp/2693190 that handled .proto files that end up in
different bazel packages. It did it by creating proto_library targets in
the correct bazel package.

Changing the granularity causes issues if the the new proto_library in
the subpackage imports a .proto file from a parent package or a
different package. e.g.
```
tmp
├── foo.proto
└── subdir/import_foo.proto # contains an `import "foo.proto"`
└── subdir/Android.bp # package boundary
├── Android.bp # contains a cc_library with foo.proto and
# subdir/import_foo.proto
```

At ToT, the ProtoInfo we provide to CcProtoGen is correct, but the
proto_library in subdir/BUILD will not compile because it does not have
a dep on the proto_library in ./BUILD

This CL creates a workaround by adding `manual` to the proto_library
targets. This CL is based on the assumption that the buildable unit in
bp2build is cc_library_*, and not proto_library necessarily (atleast
till we do a manual/automated cleanup)

Test: Created an integration test in build/bazel
Test: go test ./bp2build
Bug: 292583584
Bug: 246997908
Change-Id: I73120be2411967cb144f37ed4417f76ecf1a6ffa

bp2build/cc_library_conversion_test.go

@@ -4967,6 +4967,7 @@ cc_library_static {
 	tc.ExpectedBazelTargets = []string{
 		MakeBazelTarget("proto_library", "foo_proto", AttrNameToString{
 			"srcs": `["//bar:bar.proto"]`,
+			"tags": `["manual"]`,
 		}),
 	}
 	runCcLibraryTestCase(t, tc)
@@ -4976,6 +4977,7 @@ cc_library_static {
 	tc.ExpectedBazelTargets = []string{
 		MakeBazelTarget("proto_library", "foo_proto", AttrNameToString{
 			"srcs": `["//baz/subbaz:baz.proto"]`,
+			"tags": `["manual"]`,
 		}),
 	}
 	runCcLibraryTestCase(t, tc)
@@ -5035,6 +5037,7 @@ cc_library_static {
 			"srcs":                `["//bar:bar.proto"]`,
 			"strip_import_prefix": `""`,
 			"import_prefix":       `"bar"`,
+			"tags":                `["manual"]`,
 		}),
 	}
 	runCcLibraryTestCase(t, tc)
@@ -5046,6 +5049,7 @@ cc_library_static {
 			"srcs":                `["//baz/subbaz:baz.proto"]`,
 			"strip_import_prefix": `""`,
 			"import_prefix":       `"baz/subbaz"`,
+			"tags":                `["manual"]`,
 		}),
 	}
 	runCcLibraryTestCase(t, tc)