Fix resolution of embedded interfaces in aux files

[notbdu]

Embedded interfaces in aux_files generate unknown embedded interface XXX errors.
See below for example of the problem:

// source
import (
    alias "some.org/package/imported"
)

type Source interface {
    alias.Foreign
}

// some.org/package/imported
type Foreign interface {
    Embedded
}

type Embedded interface {}

Attempting to generate a mock will result in an unknown embedded interface Embedded.
The issue is that the fileParser stores auxInterfaces underneath the package name
explicitly specified in the aux_files flag.

In the parseInterface method, there is an incorrect assumption about an embedded interface
always being in the source file.

case *ast.Ident:
        // Embedded interface in this package.
        ei := p.auxInterfaces[""][v.String()]
        if ei == nil {
                return nil, p.errorf(v.Pos(), "unknown embedded interface %s", v.String())
        }

Test case verifies that the patch fixes the problem described in the bug report.

[pasztorpisti]

seems to be related to #85 and #10

[notbdu]

@pasztorpisti thanks for referencing those issues

[wsong]

This PR breaks this repo on Windows; aux is not a valid directory name in Windows (https://social.technet.microsoft.com/Forums/Azure/en-US/d1cf7394-a39b-46bf-823d-46f33b2f78d0/unable-to-create-folder-named-aux-anywhere-in-the-disk?forum=w7itprogeneral), so this PR prevents you from checking out this repo on a Windows machine.

[notbdu]

@wsong thanks for bringing it up. Opened a second PR w/ aux renamed to faux.