STACK_LOCK lockfile support in Stack

[Michael Snoyman]

Hi all,

There's a feature which has been available in Stack for a while now: respecting a `STACK_LOCK` environment variable to cause some locking to occur while using Stack. This feature is off by default, and I don't believe is very well documented. I'm also uncertain if it's used at all, or—with the approach to shared resources we've had in Stack for the past few years—the feature is still relevant.

I believe the code as it stands right now may be broken and/or incomplete, and play poorly with other features like Docker and Nix support. I'm putting out this email to determine:

* Is there anyone out there using this support?

* If someone is using this support, would you be able to provide clarification of what you want from this support so we can document, implement, and test it correctly?

My plan currently is, if I don't hear back from anyone requesting this support, I'll be removing it from `master` for the next release.

Relevant issue: https://github.com/commercialhaskell/stack/issues/4636

Michael