stable null safety release (#17)

drop package_resolver dependency and release 2.x version

### Breaking changes

* Dropped the dependency on `package_resolver`.
  * All `PackageResolver` apis now take a `Map<String, Uri>` of package name
    to the base uri for resolving `package:` uris for that package.
  * Named arguments have been renamed from `resolver` to `packageMap`.

Initial implementation.