Add os.path.expanduser call to normalize_path.

realpath by itself won't expand tildes, which could lead to things like:

pip wheel --wheel-path=~/wheels -r requirements.txt

Producing:

Destination directory: /home/foo/src/dir/~/wheels

Which is pretty much never what you wanted.
Merge pull request #1028 from emonty/develop

Add os.path.expanduser call to normalize_path.