Delay allocating variationMaps until populating them

Modules are created with a nil variantionMap, which avoids allocating
an empty map if the module will never be split.
Context.addVariationDependencies constructs a variationMap to
compare against the variationMap of each variant, but constructed
an empty variationMap if no variations were specified.  A nil
map is not the same as an empty map, so consistently use a nil
map and create it when populating the first entry.

Change-Id: I48b604659f9cdb23326b504a093cdfe5a3eb4f68

context.go

@@ -242,6 +242,9 @@ type Variation struct {
 type variationMap map[string]string
 
 func (vm variationMap) clone() variationMap {
+	if vm == nil {
+		return nil
+	}
 	newVm := make(variationMap)
 	for k, v := range vm {
 		newVm[k] = v
@@ -1178,6 +1181,9 @@ func (c *Context) createVariations(origModule *moduleInfo, mutatorName string,
 		}
 
 		newVariant := origModule.variant.clone()
+		if newVariant == nil {
+			newVariant = make(variationMap)
+		}
 		newVariant[mutatorName] = variationName
 
 		m := *origModule
@@ -1521,10 +1527,11 @@ func (c *Context) addVariationDependency(module *moduleInfo, variations []Variat
 	var newVariant variationMap
 	if !far {
 		newVariant = module.dependencyVariant.clone()
-	} else {
-		newVariant = make(variationMap)
 	}
 	for _, v := range variations {
+		if newVariant == nil {
+			newVariant = make(variationMap)
+		}
 		newVariant[v.Mutator] = v.Variation
 	}
 

module_ctx.go

@@ -838,6 +838,9 @@ func (mctx *mutatorContext) createVariations(variationNames []string, local bool
 	for i, module := range modules {
 		ret = append(ret, module.logicModule)
 		if !local {
+			if module.dependencyVariant == nil {
+				module.dependencyVariant = make(variationMap)
+			}
 			module.dependencyVariant[mctx.name] = variationNames[i]
 		}
 	}