Rename the main library to "http2.dart"

[nex3]

It's a strong convention among Dart packages to have a top-level library with the same name as the package (usually as the only top-level library). It's confusing that this package is named http2 but the top-level library is named transport.dart rather than http2.dart.

[mkustermann]

This was intentional. The transitive closure of transport.dart can be made independent of dart:io if necessary. The package might nonetheless provide some helpers which use features of dart:io (e.g. Socket, ...). So http2.dart could re-export transport.dart and other helpers. (see e.g. the comment Would it be possible for this to be implemented in such a way as to optionally support use with the Chrome sockets API?)

On the other hand, in 99% of cases one doesn't care, so we could also rename it.

[nex3]

Maybe the best thing to do would be to rename it for now, and then if it becomes necessary to have an IO-free library in the future re-add transport.dart and export it from http2.dart.

[sethladd]

Let's pick the best name, without worrying too much about cases like Chrome sockets APIs.