Support separately elaborating definition and instance in ChiselStage (backport #2512) (#2520)

* Support separately elaborating definition and instance in ChiselStage (#2512)

(cherry picked from commit a0aa4d15

)

# Conflicts:
#	core/src/main/scala/chisel3/experimental/hierarchy/Definition.scala
#	core/src/main/scala/chisel3/experimental/hierarchy/Instance.scala
#	core/src/main/scala/chisel3/internal/Builder.scala

* fixing imports (#2522)

Co-authored-by: Deborah Soung <debs@sifive.com>