Adds SpackCommand class allowing Spack commands to be easily in Python
Example usage:
from spack.main import SpackCommand
info = SpackCommand('info')
out, err = info('mpich')
print(info.returncode)
This allows easier testing of Spack commands.
Also:
* Simplify command tests
* Simplify mocking in command tests.
* Simplify module command test
* Simplify python command test
* Simplify uninstall command test
* Simplify url command test
* SpackCommand uses more compatible output redirection
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