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.
In this commit, we create a new package to house the ECDSA-specific
logic in the new `btcec/v2` pacakge. Thsi c hange is meant to mirror the
structure of the `dcrec` package, as we'll soon slot in our own custom
BIP-340 implementation.
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.
