Add OtherModuleExists

Allow dependency modifying mutators to check if another module exists
based on its name.  Allows primary builders to have custom handling for
missing-module errors, for example by falling back to another module.

Change-Id: I04ddd66be43c514f1f8928e15b2a79cadebcfc17

module_ctx.go

@@ -456,6 +456,7 @@ type mutatorContext struct {
 type baseMutatorContext interface {
 	BaseModuleContext
 
+	OtherModuleExists(name string) bool
 	Module() Module
 }
 
@@ -651,6 +652,10 @@ func (mctx *mutatorContext) AddInterVariantDependency(tag DependencyTag, from, t
 	mctx.context.addInterVariantDependency(mctx.module, tag, from, to)
 }
 
+func (mctx *mutatorContext) OtherModuleExists(name string) bool {
+	return mctx.context.moduleNames[name] != nil
+}
+
 // SimpleName is an embeddable object to implement the ModuleContext.Name method using a property
 // called "name".  Modules that embed it must also add SimpleName.Properties to their property
 // structure list.