Refactor circt into src/main/scala/circt, NFC

Move the git-subtree added sifive/chisel-circt repository from "circt/"
into "src/main/scala/circt/" and "src/test/scala/circtTests/".  Update
the chisel-circt README, which now lives in the src directory, to not
indicate that this is a published dependency anymore.

Signed-off-by: Schuyler Eldridge <schuyler.eldridge@sifive.com>