Better name selecting for upgrade command

[krizzu]

Summary:

Small change regarding upgrade command.
Before, if name value in package.json has changed since you init'ed a project, react-native upgrade would fail. This is because directory paths in diffing were selected based on that name value.

Now, we check if names (from package.json and app.json) are different, and if so, use the one from app.json (to keep name value in package.json untouched).

Test Plan:

Probably green CI should be sufficient.

[thymikee]

Can you try using link utilities to infer the project name? We're deprecating app.json and would like not to use it :)

[krizzu]

Closing in favor of #299