Extensible snapshots: git repos not shared by snapshot

[dbaynard]

Following from @mgsloan in #3431 I can confirm that git repos are not being shared by projects on the same machine using the same snapshot. Opening a new issue as suggested, though this relates to #3535 and #3330 too.

To reproduce; using any snapshot containing git repos,

1. Build any project using that snapshot. The git repos will be stored in the local .stack-work directory.
2. Build any other project using that snapshot. The git repos will be re-downloaded, and stored in that local .stack-work directory.

Alternatively,

1. Install a package using the global-project where the global stack.yaml refers to that snapshot. The git repos will be stored in the global-project's .stack-work directory.
2. Build a project using the same snapshot. The git repos in that project's stack.yaml will be downloaded; then all the repos from the snapshot will be re-downloaded. All will be stored in the local .stack-work directory.

I would expect that snapshot git repos would be downloaded in the global snapshot directory, once per snapshot.

My workaround involves manually linking the .stack-work/download directory of the project to that of the global-project; this is potentially problematic, as that means all git repos are downloaded there (and so two projects using different commits from the same repo will be incompatible) though I haven't run in to this issue.

Furthermore, it may also be desirable to download the stack.yaml extra-deps repos to a global directory and then check them out using 'git worktree' if available.

[mgsloan]

Thanks for looking into it! Interesting workaround too

[snoyberg]

Forgot to comment: this is already solved on the pantry branch, and I'm going to be opening up a PR to merge that code to master shortly.