PIP package names must be case insensitive, and must consider hyphens
and underscores to be equivalent
(https://www.python.org/dev/peps/pep-0426/#name), because of this the
module didn't work correctly in check mode. For example if the passed
package name had a different case or an underscore instead of a hyphen
(or the other way around) compared to the installed package, check mode
reported as changed, even though packages were installed. Now the module
ignores case and hyphens/underscores in package names, so check mode
works correctly.
