In this commit, we add a new function `RawTxInTapscriptSignature` that
will be used to generate signatures in the _tapscript_ context. Note
that this differs from top-level taproot as a distinct sighash is used,
and we _always_ accept a root hash to perform the proper tweak.
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.
