Find or install a python package, aka python lookup

First checks if the given python package is already available. If not, it first tries to install it using pip, and then using easy_install.

include(PythonPackageLookup)
lookup_python_package(<package> # Package name
    [QUIET]
    [REQUIRED]
    [VERSION <version>]         # Version to check for
                                # Not currently honoured when installing.
    [PATH <path>]               # Directory where the package should be installed
                                # Defaults to ${EXTERNAL_ROOT}/python
)