Add multi-package support to rez-pip

Extend rez-pip to support multi-package installations using both requirements.txt and adding multiple packages to the cmdline. e.g.

`rez-pip -i -r arrow Click`

I am unsure how to best handle requirements.txt files. The original argument is -r which rez-pip interprets as "release" already. Opinions?

Start at rez-pip's command-line argument handling and trace how single-package installs and the existing -r release option are parsed. Compare the requested multiple-package invocation with requirements.txt handling; done means both installation forms work without ambiguity around -r.

Written by the indexing model from the issue text.