No support for auto-downloading modules if server if running from source

[Cervator]

I expect this is because the auto-download in the server expects to find module jar files under /modules rather than source directories. I would like for us to catch that situation better and log it, then maybe attempt to find built jars (modules/[module]/build/libs/[module-[version].jar]) if present.

The logging would need to include a disclaimer that if the goal is to have a server running from source successfully serving modules the developer would have to run gradlew jar first to make sure there are actually built jar files present.

To clarify: just improving the logging sufficiently would be enough to close this issue.

Edit: Discovered this while testing #2379 which has some related stuff.

[TheFlash98]

Hey @Cervator can you suggest some place to start looking?

[Cervator]

You found it before I had a chance, nice work :)

Assigning this to you since there is a pending PR with another question open.