This adds module support for the versioned go toolchain. In particular,
the following packages are defined as modules:
* addrmgr
* blockchain
* certgen
* chaincfg
* connmgr
* database
* dcrjson
* dcrutil
* gcs
* hdkeychain
* mempool
* mining
* peer
* rpcclient
* txscript
* wire
It does not update the travis build environment or README since it is
experimental at this point.
