3.x: Add module-info.java

[ZacSweers]

This uses basic Gradle APIs to configure an MR-Jar with a module-info.java file. Resolves #7240, continuation from #7241 but without the need for an extra plugin and works around the bnd issue (they lack support for this and seeing as the issue is 4 years old it's not clear that they ever will).

This uses Gradle toolchains for simpler control over compilations, allowing us to target Java 8 for the main java sources and use Java 11 (but targeting java 9) for the module-info.java file, then adding the compiled module-info.class to the final jar under the appropriate META-INF/versions/9 path.

[codecov]

Codecov Report

Merging #7332 (21bed9d) into 3.x (b476914) will decrease coverage by 0.04%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##                3.x    #7332      +/-   ##
============================================
- Coverage     99.55%   99.51%   -0.05%     
+ Complexity     6781     6778       -3     
============================================
  Files           751      751              
  Lines         47482    47482              
  Branches       6378     6378              
============================================
- Hits          47271    47252      -19     
- Misses           96      103       +7     
- Partials        115      127      +12     

Impacted Files	Coverage Δ
.../operators/flowable/FlowableBlockingSubscribe.java	93.02% <0.00%> (-4.66%)	⬇️
...vex/rxjava3/internal/operators/maybe/MaybeAmb.java	96.96% <0.00%> (-3.04%)	⬇️
...rnal/operators/flowable/FlowableFlatMapSingle.java	94.76% <0.00%> (-2.91%)	⬇️
.../operators/observable/ObservableFlatMapSingle.java	94.44% <0.00%> (-2.39%)	⬇️
...o/reactivex/rxjava3/processors/AsyncProcessor.java	97.91% <0.00%> (-2.09%)	⬇️
...ernal/operators/flowable/FlowableFlatMapMaybe.java	96.37% <0.00%> (-2.08%)	⬇️
...va3/internal/operators/parallel/ParallelRunOn.java	98.53% <0.00%> (-1.47%)	⬇️
...internal/operators/flowable/FlowableSwitchMap.java	98.58% <0.00%> (-1.42%)	⬇️
...a3/internal/operators/flowable/FlowableReplay.java	98.71% <0.00%> (-1.08%)	⬇️
...va3/internal/operators/flowable/FlowableCache.java	99.24% <0.00%> (-0.76%)	⬇️
... and 9 more

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[akarnokd]

It's no good. Eclipse gets confused by the module-info.java

IntelliJ also complains

[akarnokd]

I suggest #7333.

[akarnokd]

Thanks for the contribution. After exploring the options, I prefer #7241 because it doesn't confuse IDEs and requires only minimal changes to the build.

[ZacSweers]

That's strange, I did this in IntelliJ and it worked fine