fix: error when importing from an extension unless enabled

[imorland]

Fixes #0000

Changes proposed in this pull request:

Reviewers should focus on:

Screenshot

Necessity

• Has the problem that is being solved here been clearly explained?
• If applicable, have various options for solving this problem been considered?
• For core PRs, does this need to be in core, or could it be in an extension?
• Are we willing to maintain this for years / potentially forever?

Confirmed

• Frontend changes: tested on a local Flarum installation.
• Backend changes: tests are green (run composer test).
• Core developer confirmed locally this works as intended.
• Tests have been added, or are not appropriate here.

Required changes:

• Related documentation PR: (Remove if irrelevant)

[dsevillamartin]

Would it make sense to warn in debug mode if the extension is NOT enabled? Just instead of failing silently - that way there's a message at all. Ideally just once per calling extension and not per import statement. Just a thought.