Closes #8

Remove deprecated `packageRoot` argument, nothing should be using this.

Change `sdkUri` to be statically a `Uri` for more type safety. We only
ever really need the String representation, but by enforcing the type we
keep usage more on the rails. The only usage in `package:test_core` is
passing a Uri, and other uses can easily wrap with `Uri.parse`.

Bump the minimum SDK to `2.4.0`. Not version of `package:test` will
support this package with a lower SDK bound, so it is not useful to
claim support for earlier SDKs that we can't test on.