You can’t use local import when specifying a non-local package to go install. If you want the local import to work, first change working directory to src/jacob.uk.com then execute go install (without specifying the package).
Of course having the helloworld.go you provided you will get an compile error: imported and not used. But once you use something from the imported greeting package, it should compile.
But you shouldn’t use local imports at all. Instead write:
import "jacob.uk.com/greeting"
And doing so you will be able to compile/run/install it from anywhere.