Retrieve licenses from other environment

Somehow related to #107

Currently, we can only get packages from the environments we are running pip-licenses on. This is a problem for two reasons

As said in mentioned issue, we cannot use it as a pre-commit hook easily. There is the possibility to use a hook within the activated environment, but it's clearly not following its guidelines
Using this libraries implies installing it in your venv, and thus contaminating it by installing packages you wouldn't need otherwise. It's no great given the purpose of this lib is to identify licenses of installed packages

I think there is a simple fix to this.

Indeed, thepckages are discovered via importlib distributions() function. As it can be seen here : https://github.com/raimon49/pip-licenses/blob/master/piplicenses.py#L193

By looking at the documentation and code of importlib, it seems to me that distributions should accept a path argument with a list of folders to search into. See https://importlib-metadata.readthedocs.io/en/latest/api.html#importlib_metadata.distributions

distributions(**kwargs) calls Distribution.discover(**kwargs) which expects either a Context object or its args for creating one (see https://github.com/python/importlib_metadata/blob/700f2c7d74543e3695163d5487155b92e6f04d65/importlib_metadata/__init__.py#L387 )

All in all, calling distributions([folder_path]) will retrieve all packages metadata in the given folder_path which could be a CLI parameter.

What do you think ?

raimon49 / pip-licenses