BSP: Create multiple BSP build targets

[stuhood]

Currently, Pants exposes a single BSP "build target" per "group" (set of Pants specs) in the config file. Because the unit of test (see #15050) is the "build target", that means that our current execution granularity for that command is "an entire set of specs".

To resolve this, we discussed (in #14943) moving group selection into the bsp command (so that the IntelliJ UI no longer needs to be used to render the groups), and then creating build targets from the transitive dependencies of the group/specs.

IntellIJ cannot just use Pants targets directly as build targets (although VSCode/Metals might be able to), because it doesn't support having multiple build targets owning a single directory. To resolve this, we should use a variant of the CoarsenedTarget algorithm (possibly via a flag to those @rules) which would execute alternating rounds of grouping by directories and coarsening to ensure both 1) no cycles, 2) non-overlapping directories for generated "build targets"/CoarsenedTargets.

[stuhood]

I'll be looking at this one in the context of figuring out transitive dependencies and resources before completing #15050.

[stuhood]

This isn't related to getting resources working, apparently. There is a separate buildTarget/resources for that.

[stuhood]

As described on #15390, when we implement this issue, we'll need to find a deterministic nearby BuildTarget to attach resources to, both in terms of ResourcesRequest, but also by including the resources's classpath entries in the class_directory of the BuildTarget. This is accomplished in #15390 by including all transitive dependencies of a BuildTarget, but that will of course need to be removed to resolve this issue.