The doc formatting changes introduced in the recent go version is
increasing the diff for all of the new commits. Formatting it all in
this commit will help the readability of future PRs by reducing the
diff.
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.
