Functions equivalent to testinfra.PipPackage.* are missing in infraspec

[ssato]

Functions equivalent to followings are missing in infraspec.

• [x] PipPackage.get_packages
• [x] PipPackage.get_outdated_packages

see also: https://testinfra.readthedocs.io/en/latest/modules.html#pippackage

[ssato]

• [x] PipPackage.get_packages: Implemented as infraspec.package.pip.list_installed
• [x] PipPackage.get_outdated_packages: Implementedas infraspec.packge.pip.list_installed(..., outdated=True, ...)

seealso: d32477f and 73c1f5e

Also, I added some more functions related to pip:

• [x] infraspec.package.pip.get_installed: Get the info of given pip package may be installed
• [x] infraspec.package.pip.is_installed: Test if given pip package is installed