fix(regression): Basic single file support

[YDX-2147483647]

We can edit typst files (even with #import) without a workspace in v0.7, but cannot in v0.8 – v0.9.3. This PR gets the feature back.

If PackageManager tries to resolve a package and finds nothing (e.g. current is empty because there's no workspace), we create a temporary package from the file's parent directory.

• Resolves: Unusable after update #232
• Investigation: Unusable after update #232 (comment)

(I've only tested it in VS Code)

This PR is implemented as a few or_else(…), so I guess it won't break any existing functions.

Question

Can CurrentPackageError::NotFound ever reach? Should we remove it?

[nvarner]

This looks like the right fix for now. If I have time, I'll go back to do some cleanup and decide what to do with CurrentPackageError::NotFound, since I think you're right that it's now unused. However, I believe this fix is correct, and I'll try to get it in a release very soon. Thank you for debugging and patching this!