…gation

When a plugin adds extraProjects, sbt creates an auto root that aggregates
all projects but has no plugin tasks (e.g. scripted). The aggregated key
parser filtered choices by data.contains(key), so (root, scripted) was
rejected and 'scripted' failed with 'No such setting/task'.

- Add optional structure param to scopedKeySelected and select
- Use isValidForAggregate when structure is present: accept key if it
  exists in data or aggregates to keys that all exist in data
- Pass structure from scopedKeyAggregated/scopedKeyAggregatedFilter
  so root-scoped keys that exist only on aggregated projects are valid

- scopedKeySelected: structure is required; callers pass None or Some(structure)
- select: add two-arg overload for callers without structure; three-arg version
  has no default so aggregate path is explicit

…tion)

- project/extra-projects-key-aggregate: repro for sbt#4947
  Run 'check' at root; without fix: No such setting/task; with fix: runs p/check

Key aggregation at root with extraProjects+auto-root aggregate was
requiring all aggregated projects to define a task (forall). Change to
exists so a task is valid if any aggregated project defines it.

The test for sbt#4947 needs an explicit root that aggregates all projects
including z from extraProjects. This validates that check works from
root when only defined in subproject p.

When extraProjects adds projects via plugins, the auto-root is created
before other projects (defined in build.sbt) are discovered. This caused
the auto-root to only aggregate the extraProjects, not all projects.

The fix updates processAutoAggregate to add missing projects to the
auto-root's aggregation after all projects have been discovered. The fix
only applies to auto-generated roots (when hasRoot=false), not to
user-defined root projects.

Fixes sbt#4947