submit: Fix outdated PR templates with lower-case names

[abhinav]

We use the contents of possible PR template sites as the hash key for caching the PR templates we get from the remote. This was only looking at PULL_REQUEST_TEMPLATE.md names, even though pull_request_template.md is valid and accepted.

This change ensures that lower-cased versions are also considered when calculating the hash key.

To do this, we consider upper-case and lower-case variants of configured Forge template paths when calculating the cache key. Case-insensitivity of this path is considered part of the Forge contract because GitHub and GitLab both treat these as case-insensitive-ish. We can adjust this assumption if we add a forge that doesn't.

Resolves #369

[abhinav]

This change is part of the following stack:

• 371 ◀

_{Change managed by git-spice.}

[abhinav]

Hm, actually I don't like this version. GitLab Merge Request templates are case-insensitive. I'll encode this in the contract.