Repository subsetting

[timothyb89]

It would be extremely useful to have some support for repository subsetting. Right now we track all modules in a given repository, but this would be bad behavior for things like e.g. the public Helm repo. It would be nice if we could only check for updates to the modules from these repositories that we care about (or that have some reverse dependency relationship)

Probably depends on #2 since we'll only want to update these repositories every 24 hours or so.