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

mk2rbc/node.go

@@ -86,6 +86,8 @@ type inheritedDynamicModule struct {
 	path             interpolateExpr
 	candidateModules []*moduleInfo
 	loadAlways       bool
+	location         ErrorLocation
+	needsWarning     bool
 }
 
 func (i inheritedDynamicModule) name() string {
@@ -97,6 +99,10 @@ func (i inheritedDynamicModule) entryName() string {
 }
 
 func (i inheritedDynamicModule) emitSelect(gctx *generationContext) {
+	if i.needsWarning {
+		gctx.newLine()
+		gctx.writef("%s.mkwarning(%q, %q)", baseName, i.location, "Including a path with a non-constant prefix, please convert this to a simple literal to generate cleaner starlark.")
+	}
 	gctx.newLine()
 	gctx.writef("_entry = {")
 	gctx.indentLevel++