Find pip cross platform without environment variable

[LexiconCode]

Description

This pull request sys.exec_prefix to find pip or pip.exe path.
@Versatilus partial credit goes to utilizing sys.exec_prefix which inspired this fix.

Related Issue

@alexboche reported in gitter.

  | Exception from starting subprocess [None, 'search', 'dragonfly2']: argument of type 'NoneType' is not iterable
--|--
  | Error loading _caster from C:\Users\alex\Documents\Caster-master\_caster.py
  | Traceback (most recent call last):
  | File "C:\NatLink\NatLink\MacroSystem\core\natlinkmain.py", line 331, in loadFile
  | imp.load_module(modName,fndFile,fndName,fndDesc)
  | File "C:\Users\alex\Documents\Caster-master\_caster.py", line 72, in <module>
  | class MainRule(MergeRule):
  | File "C:\Users\alex\Documents\Caster-master\_caster.py", line 90, in MainRule
  | R(DependencyUpdate([pip_path, "install", "--upgrade", "castervoice"])),
  | File "C:\Python27\lib\site-packages\dragonfly\actions\action_cmd.py", line 168, in __init__
  | self._str = "'%s'" % " ".join(self.command)
  | TypeError: sequence item 0: expected string, NoneType found

Motivation and Context

sys.exec_prefix to find pip or pip.exe path instead of relying on windows environmental variable Python path. action_cmd without the path for PIP leading to above error. In addition this function is cross-platform.

How Has This Been Tested

• This has been tested by removing the Python path from windows environmental variables then restarting Windows.
• function tested on linux.
• function tested on Python 3.

Types of changes

• Bug fix (non-breaking change which fixes an issue or bug)

Checklist

• I have read the CONTRIBUTING document.
• My code follows the code style of this project.
• I have checked that my code does not duplicate functionality elsewhere in Caster.
• My code implements all the features I wish to merge in this pull request.
• All new and existing tests pass.

Maintainer/Reviewer Checklist

• Basic functionality has been tested and works as claimed.
• Code is clear and readable.