rez-pip issues finding pip executable

[nerdvegas]

Two related issues:

1

If I have a python package that provides a 'python' exe but not (eg)
python2', rez can end up using the wrong py exe:

15:22:51 INFO     Found pip-8.1.1 inside /home/ajohns/packages/python/2.7.12/package.py. Will use it with /usr/bin/python2

2

rez-pip finds pip in the following order (from docstring):
1. Search for rezified python matching python version request;
2. If found, test if pip is present;
3. If pip is present, use it;
4. If not present, search for rezified pip (this is for backwards compatibility);
5. If rezified pip is found, use it;
6. If not, fall back to rez's python installation.

However, if pip is found, but its version is <19, then rather than continue on with the search for a valid pip, it errors out with:

rez.exceptions.RezSystemError: pip >= 19 is required! Please update your pip.

I have a case where the rez installation itself has a valid pip, but rez-pip will not work because it's found a rezified pip that is <v19.

TODO

• Check for both python and python2 available from the context (if pip is being used from a context), rather than assuming that 'python2' will be supplied (on non-windows). It would probably be enough to check that the exe is within the python/pip rez package's root. Given this change, I am not sure we would require checking for Windows anymore.
• Continue in the search order if a <v19 pip is found
• Improve logging to explain steps being taken.