Fixed 'pip install --user catkin_tools'

[racko]

pip passes --user --prefix= when the user calls pip install --user,
which conflicted with the mutually_exclusive_group being used to parse
these arguments.

The mutually_exclusive_group is not necessary since pip prevents the
user from passing --user together with --prefix anyhow. By not
checking it here again, we can accept pip passing --user --prefix= to
us.

---

Example:

$ cd catkin_tools
$ pip2 install --user .
[...]
  Running setup.py install for catkin-tools ... error
    Complete output from command /usr/bin/python2 -u -c "import setuptools, tokenize;__file__='/tmp/pip-NhWIKe-build/setup.py';f=getattr(tokenize, 'open', open)(__file__);code=f.read().replace('\r\n', '\n');f.close();exec(compile(code, __file__, 'exec'))" install --record /tmp/pip-fwXLCn-record/install-record.txt --single-version-externally-managed --compile --user --prefix=:
    usage: -c [--user | --prefix PREFIX]
    -c: error: argument --prefix: not allowed with argument --user/--home
    
    ----------------------------------------
Command "/usr/bin/python2 -u -c "import setuptools, tokenize;__file__='/tmp/pip-NhWIKe-build/setup.py';f=getattr(tokenize, 'open', open)(__file__);code=f.read().replace('\r\n', '\n');f.close();exec(compile(code, __file__, 'exec'))" install --record /tmp/pip-fwXLCn-record/install-record.txt --single-version-externally-managed --compile --user --prefix=" failed with error code 2 in /tmp/pip-NhWIKe-build/

The pip code that causes this:
https://github.com/pypa/pip/blob/ff5b2013a0a9001a457034481ed918e1a3fea7ef/src/pip/_internal/commands/install.py#L195-L207

[mikepurvis]

Looks safe to me, but I guess the original check was to protect in the case of non-pip usage, eg python setup.py install --prefix path/to/usr. I don't have a sense what's at stake here, but it would be nice not to block a potentially helpful behaviour from pip.

@wjwwood Any concerns here?

[wjwwood]

@wjwwood Any concerns here?

No, I just haven't had time to test it myself. Glancing at it now, it seems reasonable, but I haven't tested it.

[mikepurvis]

I confirm from local testing the error seen and that this PR corrects it and makes pip install --user work as expected with catkin_tools. Thanks for the contribution, @racko!