Enable using these modules with version constraints

[mogul]

Adds a semver module that will calculate a tag version from this repository using NPM-style version constraints.

This does not update the main README's examples to use this new semver method; we should probably discuss how best to do that before merging, hence the draft state of the PR.

[rahearn]

I like this idea, and I think for the main README we could probably do what you're doing in semver/README.md with the locals block either saying "FILL IN CONSTRAINTS" or doing something like ~>0.7 that will get all updates up to 1.0 and we can keep that more or less constant until we have breaking changes.

[mogul]

So each stanza in the README would look something like this? (Note I'm terrible at understanding version constraint syntax, so I'm kind of guessing based on the sparse examples here. And note this module uses NPM's version constraints, not Terraform's!)

module "latest-non-breaking-version" {
  source             = "github.com/18f/terraform-cloudgov//semver"
  version_constraint = "~0.7"
}

module "database" {
  source = "github.com/18f/terraform-cloudgov//database?ref=v${module.latest-non-breaking-version.target_version}"
  # [...]
}

(I also think it might be a good time to start moving those stanzas into a README.md in each subdirectory.)

[mogul]

Oh oh, wait, you meant actually using the exact same pattern with the for_each at the top of the main README.md! OK, let me add a commit for that.

[rahearn]

Oh oh, wait, you meant actually using the exact same pattern with the for_each at the top of the main README.md! OK, let me add a commit for that.

I did mean that, but also I'm good with either way

[mogul]

@rahearn or @pauldoomgov can you approve this one? I'd like to merge it in support of moving stuff out of the FAC repo into here.

[rahearn]

Love that this is a purely optional quality of life add-on

[mogul]

Welp, uh, bad news... You cannot use this module's output as the source value for another module, because that would be an expression. Doh! Expect a PR rolling out the documentation changes.