Make MakeVarsContext a PathContext

Expose all of SingletonContext to makeVarsContext, and then export
the subset of it that is used through MakeVarsContext.SingletonContext,
plus what is necessary for PathContext, directly through
MakeVarsContext.

Test: m checkbuild
Change-Id: Ie00f36e577fe110b6fa03b901da489d8547773c6
diff --git a/cc/makevars.go b/cc/makevars.go
index d91735f..4a9ade2 100644
--- a/cc/makevars.go
+++ b/cc/makevars.go
@@ -100,7 +100,7 @@
 
 	// Filter vendor_public_library that are exported to make
 	exportedVendorPublicLibraries := []string{}
-	ctx.SingletonContext().VisitAllModules(func(module android.Module) {
+	ctx.VisitAllModules(func(module android.Module) {
 		if ccModule, ok := module.(*Module); ok {
 			baseName := ccModule.BaseModuleName()
 			if inList(baseName, vendorPublicLibraries) && module.ExportedToMake() {