Don't invalidate ExtModule ports in an explicitInvalidate = true context (backport #2795) (#2799)

* Don't invalidate ExtModule ports in an explicitInvalidate = true context (#2795)

* Don't invalidate ExtModule ports in an explicitInvalidate = true context

ExtModule ports were previously invalidated in the emitted FIRRTL, which is correct in a NonStrict / `Chisel._` compatibility context but not in newer chisel3 code where `explicitInvalidate = true`.

(cherry picked from commit 8e24a281

)

# Conflicts:
#	core/src/main/scala/chisel3/BlackBox.scala

* Resolve backport conflicts

Co-authored-by: Jared Barocsi <82000041+jared-barocsi@users.noreply.github.com>
Co-authored-by: Jack Koenig <koenig@sifive.com>