Make `.packages` files be more clever about defaults. (dart-lang/package_config#80)

* Make `.packages` files be more clever about defaults.

The `PackageConfig` for a `.packages` file now assigns a default language version of 2.7
to all packages, and if the package location ends in `/lib/`, it assumes the package's
root directory is the parent directory of that.

* Fix test.

* Fix another test.