Refactor helper function '_get_pip' to handle the cases where an absolute
path explicit executable is passed to the task and to look for an explicit executable
by basename in a virtualenv if that is specified.
Implement a '_get_easy_install' helper function to find the desired
'easy_install' executable, modeled on '_get_pip'
from the 'pip' module to correctly handle all combinations of explicit
executable and virtualenv.