Files leak across instances of Project

[lolmaus]

    let project1 = new Project('my-app', '1.0.0');
    project1.files['foo.js'] = 'foo';

    let project2 = new Project('my-app', '1.0.0');

    expect(project2.files['foo.js']).to.be.undefined; // => 💣
    project2.files['foo.js'] // => 'foo'

There's a comment about shallow merging here, but the fact is that files are stored in a module-level variable that is simply assigned to the files instance property in teh constructor,

[lolmaus]

This is likely to break a lot of tests in Embroider with scenario-tester.