Aliases fail on imported namespaces

[200sc]

Given this Magefile at site.go/project:

// +build mage

package main

import (
	// mage:import
	"site.com/project/subfolder"
)

var Aliases = map[string]interface{}{
	"fizz": subfolder.Foo.Bar,
}

And this single go file being imported from site.go/project/subfolder:

// +build mage

package subfolder

import (
	"github.com/magefile/mage/mg"
)

type Foo mg.Namespace

func (Foo) Bar() error {
	return nil
}

The following warning is received on running mage: warning: alias declaration (fizz) does not reference a mage target

Following this, mage exits with status 1.

mage -version outputs:

Mage Build Tool v1.7.1
Build Date: 2018-10-31T09:48:07-05:00
Commit: 11d1591
built with: go1.11

This was built on windows/amd64

The behavior expected would be for fizz to be a valid alias to the target in the subfolder.

[natefinch]

So, when you import from another package, the code you import can't be marked with the mage build tag. However, even without the mage build tag, it still fails with that error.

I'll make a fix, it's probably something small and easy.