Makefile: cleanups for newer versions of Go

Go 1.9 removed "./..." matching the vendor directory while Go 1.10 added
build and test caching. This means we no longer need to grep out
vendored matches (except for golint which doesn't implement the same
behavior), and we no longer need to pre-build packages with "go build -i".

https://golang.org/doc/go1.9#vendor-dotdotdot
https://golang.org/doc/go1.10#build