More refactoring for Resolve logic

[lihaoyi]

Major Changes

1. Break out main/resolve/ from main/, depending on main/define/ but not depending on main/eval/.

   1. Resolving tasks should not depend on Evaluator, and the core logic of Evaluator should not depend on task resolution (even if individual tasks might require it).
   2. Breaking it up into a separate modules enforces that, and helps simplify each module so the complex logic within each one can be reasoned about independently.

2. Minimize the amount of actual instantiations we perform in ResolveCore

   1. We can postpone instantiating Targets and Commands until resolution is complete, removing a whole bunch of messy mainargs logic for the already-messy recursive resolution code. A lot of it ends up in Resolve.scala, which is somewhat more straightforward and better able to absorb the messiness
   2. We only need to instantiate Modules when trying to resolve tasks within a Cross module, since that requires us to instantiate the Cross object to see it's cross keys. Most resolution can take place purely by reflecting on java.lang.Class[_]s without needing the modules to actually be instantiated
   3. ResolveCore now only returns the segments: Segments for the things it resolves, leaving instantiation to Resolve.Tasks
   4. This significantly increases the laziness of ResolveCore, letting mill resolve work faster and work even when the modules cannot instantiate, unless there is a Cross module we need to instantiate

3. Refactored ResolveNonEmpty to ResolveNotFoundHandler, to only be called by Resolve if ResolveCore returns a ResolveCore.NotFound. That helps avoid deep call hierarchies, and allows us to ensure that ResolveCore and ResolveNotFoundHandler independent of each other (rather than one wrapping the other)

4. Extracted mill.define.Module.Internal into a separate mill.define.Reflect object. This helps clean up Module.scala. Ideally I wanted to move Reflect to mill.resolve, but it's difficult to do that while still providing nice helper methods on mill.define.Module objects e.g. millModuleDirectChildren while allowing them to be overriden etc.

5. ResolveTasks is now Resolve.Tasks, ResolveSegments is now Resolve.Segments, ResolveMetadata is removed since Resolve.Segments does more or less the same thing anyway

6. Make Cross#items a List[Item], so we can publicize it while continuing to evolve it in a binary-compatible manner

Testing

1. Added more asserts to the existing ResolveTests.scala unit tests to check that ResolveSegments can still succeed even when ResolveTasks fails due to module initialization errors

2. Added additional test cases to check the behavior of cross-module failures in more scenarios: when _ and __ is used, when the cross module itself fails to initialize (previously we only checked for children failing to initialize), and when the cross module's parent module fails to initialize

3. Added a few test cases to integration/failure/module-init-error/ exercise the "resolve works even when module cannot initialize" behavior end-to-end

4. Existing test cases in ResolveTests.scala and downstream tests that exercise task resolution all pass

[lefou]

This looks good to me. I think, we should provide some @deprecated forwarders for the classes moved to mill.resolve in the mill.main package object. This should improve migration convenience and cross compilation.

[lefou]

@lihaoyi We have one failing test in main and it seems related to this PR. Could you please look at it?

X mill.resolve.ResolveTests.overridenModule.0 5ms 
  utest.AssertionError: resolvedTasks.map(_.map(_.toString).toSet[String]) ==
  resolvedTasks: Either[String,List[mill.define.NamedTask[Any]]] = Left(Cannot resolve sub.inner.subTarget. Try `mill resolve sub.inner._` to see what's available.)
  expected: scala.util.Either[String,scala.collection.immutable.Set[mill.define.NamedTask[Any]]] = Right(Set(sub.BaseInnerModule.subTarget))
    utest.asserts.Asserts$.assertImpl(Asserts.scala:30)
    mill.resolve.ResolveTests$Checker.checkSeq(ResolveTests.scala:24)
    mill.resolve.ResolveTests$Checker.apply(ResolveTests.scala:14)
    mill.resolve.ResolveTests$.$anonfun$tests$257(ResolveTests.scala:754)

[lihaoyi]

will take a look

[lihaoyi]

CI is green now, but I do not understand what caused the breakage, nor whay fixed it, so we'll have to see if the issue reappears. If it turns out the test is just flaky then we may be forced to disable it

[lihaoyi]

CI is green now, but I do not understand what caused the breakage, nor whay fixed it, so we'll have to see if the issue reappears. If it turns out the test is just flaky then we may be forced to disable it