Add facilities to query the association between package versions and package sets

Right now it's hard to explore the relationship between a package, its versions, and the various package sets - there are only a few facilities:

• spago ls packages will list the packages present in the package set that the project uses
• spago registry package-sets will list the package sets that have been published, together with the compiler that they are compatible with

We should improve on that, and have ways to:

1. list which packages exist in any given package set, not just the local one
2. list which package sets a certain package version is part of

For each of the points above, I propose that:

1. we extend spago registry package-sets $SET to show which packages are in the set
2. we extend spago registry info $PACKAGE with a list of which package set each version is present in.

I'll note that we already have all this info in the Spago DB, and even have code in place to query it, and most likely just needs to be exposed:
https://github.com/purescript/spago/blob/b5a8ab980010ad08ed0c305d7ff06d337d1a0e89/src/Spago/Db.purs#L70-L80

Start with the existing database query code in src/Spago/Db.purs at the linked lines, then trace the handlers for spago registry package-sets $SET and spago registry info $PACKAGE. The work is done when the first command lists packages in the requested set and the second shows the package sets containing each version.