Don’t produce duplicate warnings when invoking findAddonByName with a…

[stefanpenner]

…n ambiguous add-on name

[stefanpenner]

failures seem unrelated:

not ok 335 EmberApp vendorFiles does not include jquery if the app has `@ember/optional-features` with the `jquery-integration` FF turned off
  AssertionError: expected [ Array(3) ] to deeply equal [ 'ember.js', 'app-shims.js' ]
      at Context.<anonymous> (tests/unit/broccoli/ember-app-test.js:1314:52)

<snip>

not ok 347 EmberApp: Bower Dependencies "after each" hook: createPromise for "are not packaged unless explicitly imported"
  TypeError: Cannot read property 'dispose' of undefined
      at Context.<anonymous> (tests/unit/broccoli/ember-app/import-test.js:34:18)
      at Generator.next (<anonymous>)
      at onFulfilled (node_modules/co/index.js:65:19)

[stefanpenner]

@rwjblue the tests seem to be failing for an unrelated issue. I'll try to look into this in the AM.