introducing: per-module checks

[cdmistman]

Why

we want the testing story to be a lot better for v2 than it was for v1

this testing system allows us to write completely hermetic checks inline with the rest of the module declarations, while also allowing us to test various combinations of modules

the system is very bare-bones but i expect it to grow as we define needs

What changed

• added new check function as an output to v2/default.nix
• added new checks option

Test plan

for now, can be tested like so:

$ nix repl
> :lf github:replit/nixmodules/cad/module-checks
> :b packages.x86_64-linux.v2.check { interpreters.nodejs.enable = true; }
  out -> /nix/store/abc123-modules-checks
$ /nix/store/abc123-modules-checks

note that passing in 0 enabled modules will result in an empty checks file

Rollout

Describe any procedures or requirements needed to roll this out safely (or check the box below)

• This is fully backward and forward compatible

[cdmistman]

this test could've caught a bug in a previous release of v1!

[airportyh]

Yes, this will be very important when we enforce version pinning. Do not want to accidentally upgrade the version by upgrading a Nix channel.

[airportyh]

Could you setup a way to run all module checks as a command that can be used in ci?