[java] Download 3rd-party Java sources from Maven

[alexander-yakushev]

Finally, a way to download source JARs at runtime. Sister PRs: clojure-emacs/cider-nrepl#911, clojure-emacs/cider#3769.

I tried a few approaches and settled with this one – downloading happens within class->source-file-url if *download-sources-jar-fn* is bound. The caller is expected to bind it to something that eventually invokes download-sources-jar-for-coordinates but can do custom logic and predicates too. I'm not 100% happy with this solution, it feels kinda awkward, but this is the best I could do. I honestly tried to make it strictly "cider-nrepl invokes Orchard", but the amount of repeated work was too much. Common Lisp condition system would actually make this cleaner, but hey, that's what dynamic vars actually are in the end, right?

Different ways of downloading is supported. Ultimately, all of them involve spawning subprocesses:

• Clojure 1.12 projects with clojure build tool can invoke the new shiny invoke-tool function.
• Pre-1.12 you can subshell clojure directly.
• Leiningen projects can call lein.

Orchard doesn't do build tool detection on its own, the caller is expected to pass it explicitly.

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• You've added tests to cover your change(s)
• You've updated the changelog

[bbatsov]

I'm not 100% happy with this solution, it feels kinda awkward, but this is the best I could do. I honestly tried to make it strictly "cider-nrepl invokes Orchard", but the amount of repeated work was too much. Common Lisp condition system would actually make this cleaner, but hey, that's what dynamic vars actually are in the end, right?

Yeah, it's not work of art, but I think the current implementation is fine, and we can always polish it down the road as I doubt it will have other users besides cider-nrepl for the foreseeable future.

Perhaps it'd be good to add some section to the README explaining how clients are supposed to use *download-sources-jar-fn*.

[alexander-yakushev]

Expanded the README.

Also, moved the discovery of the underlying build tool here. It is more reliable to perform on the Clojure side.

I don't rush to merge this; I will start merging once I get approval on all three PRs.