Allow for a list of packages to install to be given as a flag

[tibbe]

There are two types of packages that are currently causing me problems:

• The "python" package appears as a dependency if you use Poetry (rather than as an entry under project.requires-python). Building Python in a venv fails for me (and is unnecessary).
• Any local dependencies will fail (e.g. using path and develop).

I've worked around this by commenting these out but it would be nice if the program had a CLI flag to exclude a list of packages by name.