Support more options for which GAP packages to download

[wilfwilson]

At least no packages, bootstrap-pkg-minimal, and (what is now the only option) bootstrap-pkg-full.

Ideally also the latest released version of each package (so, possibly newer than what you get with bootstrap-pkg-full).

PackageManager is probably also a good one to include, for use by later actions (e.g. #28).

[fingolfin]

We actually removed the option to use different package sets other than "full" in v3, because use cases were unclear and it simplifies a bunch of things later on.

If there is a good use case why we should reconsider this, I'd be interested to hear it!

CC @stertooy

[wilfwilson]

I don't remember why I made this issue.

I do like the idea of using the latest released version of each package (presumably this information should be easily-enough accessible via the PackageDistro), so that during development you can see if your changes are breaking someone else's package, or get early notification that someone has broken yours. But I get that that's quite a niche thing.