The use of the GO111MODULE environment variable doesn't have any effect
anymore and hasn't for a couple of versions. The default was set to "on"
a while back, so we can remove that variable everywhere.
The `goveralls` tool we use to handle code coverage upload seems to not
understand that a `v2 module can exist, without having a v2 file path on
disk. We use a `sed` command to remove the `v2` module prefix so the
tool can reach into the correct file to extract the source code.
In this commit, we adapt the Makefile that was being used for the
btcutil project to work for btcd as well. The Makefile is pretty simple,
and is just a series of templated commands. Overtime, we can pull in
some of the `lnd` additions as well, which we use to handle our
reproducible build and verification system.
