Build FIRRTL in-tree, not published dependency (#2983)

Change the Scala build to use the FIRRTL subproject that was just
brought in-tree.  This removes the dependency of Chisel on the published
version of FIRRTL.  This work is done in preparation to remove the
Scala-based FIRRTL Compiler (SFC) from Chisel.

Signed-off-by: Schuyler Eldridge <schuyler.eldridge@sifive.com>
Co-authored-by: Adam Izraelevitz <adam.izraelevitz@sifive.com>