Rethink alternative syntax as optional syntax extension

[seaneagan]

With #5 the alternative syntax is:

import foo:src.bar
import :src.bar

Could instead continue using ".", but just allow omission of the first segment:

import foo.src.bar
import .src.bar

[seaneagan]

Also it could be relative to the "current library" instead of the "current package", in other words, short for a path import, so:

import .src.bar;

would be short for:

import 'src/bar.dart';

In fact, I tend to always use relative paths to import libraries in the same package. I've had problems with using 'package:` imports for that, and with mixing relative and 'package:' imports.