In this commit, we update all the btcutil imports to point to the new
sub-module.
In the same commit, we also modify the recently added `btcutil/go.mod`
file as we need to continue pointing to the _old_ version of btcd, until
we merge this PR and push a new tag.
To allow using a custom btcd executable, we allow specifying a path to a
file. If the path is empty, the harness will fall back to compiling one
from scratch.
