Rework the way variant names are stored

Replace the array of mutator name/variant name pairs with
a map of mutator name to variant name, and store the string
variant name separately.

Change-Id: I181c2fcb05724c8755b90aaf866fdce9ef720b01
diff --git a/module_ctx.go b/module_ctx.go
index ac80d03..7e8c775 100644
--- a/module_ctx.go
+++ b/module_ctx.go
@@ -249,7 +249,7 @@
 }
 
 func (m *moduleContext) ModuleSubDir() string {
-	return m.module.subName()
+	return m.module.variantName
 }
 
 func (m *moduleContext) Variable(pctx *PackageContext, name, value string) {
@@ -384,11 +384,7 @@
 // Set all dangling dependencies on the current module to point to the variant
 // with given name.
 func (mctx *mutatorContext) SetDependencyVariant(variantName string) {
-	subName := subName{
-		mutatorName: mctx.name,
-		variantName: variantName,
-	}
-	mctx.context.convertDepsToVariant(mctx.module, subName)
+	mctx.context.convertDepsToVariant(mctx.module, mctx.name, variantName)
 }
 
 func (mctx *mutatorContext) Module() Module {