With the use of go modules, the btcdPkgPath will no longer be `github.com/btcsuite/btcd`, but end with a version string (e.g. btcd@v0.0.0-20181129140220-beb77e89572a).
This trips up build.ImportDir, which will return a ImportPath of ".", since this package cannot be found in the go path.
There are probably several ways of fixing this, by clever string magic, but seems easiest to just hardcode the btcd package name.
Previously, rpctest would start a btcd node using the btcd executable
in the environment PATH. This caused difficult-to-find issues where
the code would be tested against an older version of btcd, or another
fork entirely. Now it compiles btcd the first time it is needed and
uses that fresh version when launching nodes.
