Speed up unpack_file_url

by ignoring ('.tox', '.git', 'build') when doing `shutil.copytree`.

Fixes: GH-2195

##### Before

```
$ time pip install --no-install ~/dev/git-repos/pip
DEPRECATION: --no-install and --no-download are deprecated. See https://github.com/pypa/pip/issues/906.
Processing /Users/marca/dev/git-repos/pip
  Requirement already satisfied (use --upgrade to upgrade): pip==6.0.dev1 from file:///Users/marca/dev/git-repos/pip in /Users/marca/dev/git-repos/pip
pip install --no-install ~/dev/git-repos/pip  2.80s user 5.86s system 50% cpu 17.205 total
```

##### After

```
$ time pip install --no-install ~/dev/git-repos/pip
DEPRECATION: --no-install and --no-download are deprecated. See https://github.com/pypa/pip/issues/906.
Processing /Users/marca/dev/git-repos/pip
  Requirement already satisfied (use --upgrade to upgrade): pip==6.0.dev1 from file:///Users/marca/dev/git-repos/pip in /Users/marca/dev/git-repos/pip
pip install --no-install ~/dev/git-repos/pip  0.80s user 0.87s system 70% cpu 2.392 total
```

From 17 seconds to less than 3 seconds; not bad! :smile: