Merge "Check `base` property before using it"

// next phase.

func overrideModuleDepsMutator(ctx BottomUpMutatorContext) {

	if module, ok := ctx.Module().(OverrideModule); ok {

		base := String(module.getOverrideModuleProperties().Base)

		if !ctx.OtherModuleExists(base) {

			ctx.PropertyErrorf("base", "%q is not a valid module name", base)

			return

		}

		// See if there's a prebuilt module that overrides this override module with prefer flag,

		// in which case we call SkipInstall on the corresponding variant later.

		ctx.VisitDirectDepsWithTag(PrebuiltDepTag, func(dep Module) {