Enable explicitly specifying pre-releases without enabling pre-releases

[MHendricks]

Currently forcing a pre-release version to hab without enabling prereleases in the site/cli causes an InvalidRequirementError error. Hab should handle this in the same way as pip does.

Normally it should ignore pre-release versions unless you pass the --pre flag to the cli(or enable prereleases in your site). However if you pass an <=, >=: inclusive ordered comparison specifier that specific specifier, that specific specifier should enable pre-releases without affecting the other requirements.

Checklist

• I have read the CONTRIBUTING.md document
• I formatted my changes with black
• I linted my changes with flake8
• I have added documentation regarding my changes where necessary
• Any pre-existing tests continue to pass
• Additional tests were made covering my changes

Types of Changes

• Bugfix (change that fixes an issue)
• New Feature (change that adds functionality)
• Documentation Update (if none of the other choices apply)

Proposed Changes

You can replicate this issue by including the pre-release distros from this pull request. Run the command hab -r "pre-release<=1.dev9" dump not_set/empty_lists.

If you don't include the changes to distro.py you will replicate the current problem where hab raises:

hab.errors.InvalidRequirementError: Unable to find a valid version for "pre-release<=1.dev9" in versions [1.0.dev1, 1.0, 1.1.dev2]

But if you include the changes to distro.py it will resolve the distro pre-release==1.0.dev1.

This preserves the existing use of --pre. If you run hab -r "pre-release" dump not_set/empty_lists -vvv it will resolve pre-release==1.0. Adding the flag --pre will resolve the pre-release version pre-release==1.1.dev2.