Allow variable-prefixed include statements in mk2rbc

mk2rbc was already searching the whole android tree
for Makefiles, so allowing variable-prefixed include
statements doesn't affect performance on the file searching
front. On the generated code front, there's already a cap
of 150 potentially-included makefiles that prevents the
performance from getting too bad, and it can be lowered
if necessary.

Bug: 213508006
Test: go test
Change-Id: I3a4e81acb3d97bee08ac3dbe63052a274acf5793
diff --git a/mk2rbc/mk2rbc.go b/mk2rbc/mk2rbc.go
index e317cad..ebb463b 100644
--- a/mk2rbc/mk2rbc.go
+++ b/mk2rbc/mk2rbc.go
@@ -829,11 +829,7 @@
 			pathPattern = append(pathPattern, chunk)
 		}
 	}
-	if pathPattern[0] == "" {
-		if len(ctx.includeTops) == 0 {
-			ctx.errorf(v, "inherit-product/include statements must not be prefixed with a variable, or must include a #RBC# include_top comment beforehand giving a root directory to search.")
-			return
-		}
+	if pathPattern[0] == "" && len(ctx.includeTops) > 0 {
 		// If pattern starts from the top. restrict it to the directories where
 		// we know inherit-product uses dynamically calculated path.
 		for _, p := range ctx.includeTops {
@@ -849,14 +845,20 @@
 		ctx.errorf(v, "there are >%d files matching the pattern, please rewrite it", maxMatchingFiles)
 		return
 	}
-	res := inheritedDynamicModule{*varPath, []*moduleInfo{}, loadAlways}
-	for _, p := range matchingPaths {
-		// A product configuration files discovered dynamically may attempt to inherit
-		// from another one which does not exist in this source tree. Prevent load errors
-		// by always loading the dynamic files as optional.
-		res.candidateModules = append(res.candidateModules, ctx.newDependentModule(p, true))
+	if len(matchingPaths) == 1 {
+		res := inheritedStaticModule{ctx.newDependentModule(matchingPaths[0], loadAlways && ctx.ifNestLevel == 0), loadAlways}
+		processModule(res)
+	} else {
+		needsWarning := pathPattern[0] == "" && len(ctx.includeTops) == 0
+		res := inheritedDynamicModule{*varPath, []*moduleInfo{}, loadAlways, ctx.errorLocation(v), needsWarning}
+		for _, p := range matchingPaths {
+			// A product configuration files discovered dynamically may attempt to inherit
+			// from another one which does not exist in this source tree. Prevent load errors
+			// by always loading the dynamic files as optional.
+			res.candidateModules = append(res.candidateModules, ctx.newDependentModule(p, true))
+		}
+		processModule(res)
 	}
-	processModule(res)
 }
 
 func (ctx *parseContext) findMatchingPaths(pattern []string) []string {