In this commit, we update the top level module to use the newly tagged
sub-modules. Once we remove the circular dependant in these sub-modules,
then we'll no longer have to do things like this.
This was the version tagged that created the new `ecdsa` and `schnorr`
packages. Updating these pinned version lets importers properly use
these packages and build. Things build as is since we use replace
directive to point to the latest version _in the repo_ when we build.
Now that the new `btcutil/v1.0.0` tag has been pushed, we update the new
internal modules to point to the top-level `btcd` repo via replace
directives.
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.
