Add syntactic sugar for package imports about language

[Wdestroier]

You may want to upvote issue #649.

[lrhn]

This is effectively asking for less than #649, that the URI package:foo is accepted and treated as equivalent to (or normalized to) package:foo/foo.dart.

It's not impossible. It requires adding a normalization step to package URI parsing, which adds the /foo.dart, but it should be well defined how to do that.

As in #649, a package:foo.bar.baz would normalize to package:foo.bar.baz/baz.dart, to support existing, and hypothetical future, uses of dotted package names.

I want to normalize the URI rather than having two different URIs denote the same file, because Dart library identity is defined by URI equality.
Alternatively we change the rules for library identity to allow equivalences on URIs, so the compiler gets to decide when two URIs are equivalent (within reason).