Support tagged module references

There are cases where a module needs to refer to an intermediate
output of another module instead of its final output.  For example,
a module may want to use the .jar containing .class files from
another module whose final output is a .jar containing classes.dex
files.  Support a new ":module{.tag}" format in any property that
is annotated with `android:"path"`, which will query the target
module for its ".tag" output(s).

Test: path_properties_test.go, paths_test.go
Test: no unexpected changes in build.ninja
Change-Id: Icd3c9b0d83ff125771767c04046fcffb9fc3f65a
diff --git a/java/droiddoc.go b/java/droiddoc.go
index 57da6b6..0ec7799 100644
--- a/java/droiddoc.go
+++ b/java/droiddoc.go
@@ -497,8 +497,13 @@
 	stubsSrcJar android.WritablePath
 }
 
-func (j *Javadoc) Srcs() android.Paths {
-	return android.Paths{j.stubsSrcJar}
+func (j *Javadoc) OutputFiles(tag string) (android.Paths, error) {
+	switch tag {
+	case "":
+		return android.Paths{j.stubsSrcJar}, nil
+	default:
+		return nil, fmt.Errorf("unsupported module reference tag %q", tag)
+	}
 }
 
 func JavadocFactory() android.Module {
@@ -519,7 +524,7 @@
 	return module
 }
 
-var _ android.SourceFileProducer = (*Javadoc)(nil)
+var _ android.OutputFileProducer = (*Javadoc)(nil)
 
 func (j *Javadoc) sdkVersion() string {
 	return String(j.properties.Sdk_version)