Rename rez.vendor.version to rez.version and define a public API for rez.version

[JeanChristopheMorinPerso]

Implements #1484.

This PR renames/moves rez.vendor.version out of the vendor folder into its new home, rez.version. This makes it a first class citizen.

Changes

• rez.vendor.version moved to rez.version
• All its submodules are now private:
  • rez.vendor.version.version renamed to rez.version._version
  • rez.vendor.version.util renamed to rez.version._util
  • rez.vendor.version.requirement renamed to rez.version._requirement
• rez.version re-exports all classes and functions that are public
• Some functions that were previously public are now private:
  • rez.vendor.version.util.dedup
• Added a deprecation warning in rez.vendor.version that loks like this:

  (.venv) [jcmorin@arch01 rez]$ python -c 'import rez.vendor.version'
  <string>:1: DeprecationWarning: module 'rez.vendor.version' is deprecated and will be removed in 3.0.0. Use 'rez.version' instead.
  

Notes

#1562 should be merged before so that we can use rez.deprecations.

TODOs

• Properly document (unify all module docstrings into rez.version).
• Update docstrings to be valid Sphinx docstrings.