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

java/java.go

@@ -351,15 +351,20 @@ type Module struct {
 	dexpreopter
 }
 
-func (j *Module) Srcs() android.Paths {
-	return append(android.Paths{j.outputFile}, j.extraOutputFiles...)
+func (j *Module) OutputFiles(tag string) (android.Paths, error) {
+	switch tag {
+	case "":
+		return append(android.Paths{j.outputFile}, j.extraOutputFiles...), nil
+	default:
+		return nil, fmt.Errorf("unsupported module reference tag %q", tag)
+	}
 }
 
 func (j *Module) DexJarFile() android.Path {
 	return j.dexJarFile
 }
 
-var _ android.SourceFileProducer = (*Module)(nil)
+var _ android.OutputFileProducer = (*Module)(nil)
 
 type Dependency interface {
 	HeaderJars() android.Paths
@@ -813,8 +818,6 @@ func (j *Module) collectDeps(ctx android.ModuleContext) deps {
 			}
 		default:
 			switch tag {
-			case android.DefaultsDepTag, android.SourceDepTag:
-				// Nothing to do
 			case systemModulesTag:
 				if deps.systemModules != nil {
 					panic("Found two system module dependencies")
@@ -824,8 +827,6 @@ func (j *Module) collectDeps(ctx android.ModuleContext) deps {
 					panic("Missing directory for system module dependency")
 				}
 				deps.systemModules = sm.outputFile
-			default:
-				ctx.ModuleErrorf("depends on non-java module %q", otherName)
 			}
 		}
 	})