Fix a "Duplicated library name 'Uuid'." Dart2JS warning

The cause of the warning was that previously
both uuid.dart and uuid_client.dart declared
"library uuid;".

This commit changes the package structure to
make it work more like the "chrome" package,
i.e. move uuid.dart out of the way into a
subfolder and declare "library uuid_server;"
and "library uuid_client;" in the respective
files.