`wire-cell-toolkit` now softly depends on `wire-cell-python`

[brettviren]

Recent work on wire-cell-toolkit tests have formed a soft dependency on wire-cell-python which should be reflected in the Spack packaging.

This is a "soft" dependency in that the BATS scripts which wish to call wirecell-* CLIs from w-c-p will warn but otherwise pass if they are not available.

The packaging should be updated to assert this dependency as an option that defaults to "on".