Support build-time dependencies for jank-build.bb scripts.

[kylc]

Adds support for build dependencies: dependencies for your jank-build.bb script. For example, a common jank-build-cmake library which includes helpers for executing CMake, to be used by all jank-build.bb scripts in the wild which drive a CMake build.

This mimics the Cargo build-dependencies feature.

I have updated my lein-jank-playground to demo this:

1. jank-json-sys declares a :build-dependencies entry in its project.clj file
2. It uses the dependency in its jank-build.bb file
3. This dependency is resolved via Maven as usual, in this case to an artifact on clojars (source: https://github.com/kylc/lein-jank-playground/tree/main/jank-build-cmake). It is added to the babashka classpath when the build script is executed.

Also includes some misc. fixups and some integration tests for the build system.

TODO

• Validate (build?)-dependencies-of-build-dependencies behavior
• Think about whether using this non-standard pom.xml dependency scope jank-build will cause any problems down the line

[jeaye]

This looks great. I will ask technomancy if he sees any issue with the scope usage for build deps.

[kylc]

Got rid of the exit calls here because some standard lein tasks call out to these unconditionally. For example lein install seems to always lein compile (that makes sense for building JVM JARs). In our case, if a library does not have a main, this will cause lein install to abort, which we don't want.

[kylc]

There is a bit more to figure out with the fingerprinting to make sure rebuilds trigger whenever something changes. Consider this a mostly functional placeholder for now.

[kylc]

This looks great. I will ask technomancy if he sees any issue with the scope usage for build deps.

It's nice because the scope fits into the standard leiningen dependency tree functionality, so we can easily pick out build dependencies without any additional code. But if it's a no-go then we can implement our own logic to read the dependencies' project.clj and pull out the build-dependencies key.

[kylc]

This is ready for review. I shoehorned some other improvements into this same PR, but they are somewhat organized as separate commits.

[jeaye]

I asked technomancy and some others, on IRC, and heard back that our usage of dependency scopes here should be just fine.

[jeaye]

Looks great to me! Just hoping to get the tests into CI.

[jeaye]

Would you mind hooking this into CI for this PR? I think we'll need a new job for the lein-jank project. Maybe .github/workflows/test-lein-jank.yml.

Otherwise, I'll never remember to run this.