Update @ember/string detection to work on itself

[locks]

When running the tests for @ember/string, ember-cli-babel does not detect it properly and tries to process the imports for that module, leading to a failed test run.

[rwjblue]

Hmmm. Isn't the fix needed here to actually tweak _emberStringDependencyPresent to be something like:

_emberStringDependencyPresent() {
  if (this.project.name() === '@ember/string') {
    return true;
  }

  let checker = new VersionChecker(this.parent).for('@ember/string', 'npm');	    
  return checker.exists();
},

[locks]

@rwjblue 🤦‍♂️ updated.