lib: Move to go modules and shed a few tears

The old system with vendor/ and git submodules worked great,
unfortunately FUD around git submodules seemed to scare people away and
golang moved to a go.mod system that adds a new lock file format instead
of using the built-in git version. It's now almost impossible to use
modern golang without this, so we've switched.

So much for the golang compatibility promise-- turns out it doesn't
apply to the useful parts that I actually care about like this.

Thanks to frebib for his incredibly valuable contributions to this
patch. This snide commit message is mine alone.

This patch also mixes in some changes due to legacy golang as we've also
bumped the minimum version to 1.16 in the docs and tests.

Lastly, we had to disable some tests and fix up a few other misc things
to get this passing. We've definitely hot bugs in the go.mod system, and
our Makefile tries to workaround those.

etcd/client/strmap/strmap.go

@@ -26,8 +26,8 @@ import (
 	"github.com/purpleidea/mgmt/util"
 	"github.com/purpleidea/mgmt/util/errwrap"
 
-	etcd "go.etcd.io/etcd/clientv3"
-	etcdutil "go.etcd.io/etcd/clientv3/clientv3util"
+	etcd "go.etcd.io/etcd/client/v3"
+	etcdutil "go.etcd.io/etcd/client/v3/clientv3util"
 )
 
 const (