This CL got away from me a little.

For a number of reasons, the existing goimports API of passing in values
for various GO* values was not working. For one, the number of necessary
variables kept growing. For another, we tried to avoid calling `go env`
in GOPATH mode by using `build.Default`, but that turns out to be buggy;
see golang/go#39838. And finally, it created massive confusion about
whether the values were intended to be read from the OS environment, or
fully evaluated by the `go` command.

There are only two users of the internal imports API, so there really
shouldn't need to be more than two modes. For the command line tool, we
have to call `go env` to deal with the `go/build` bug. So we just do it.
Tests use that same path, but can augment the enviroment to set
themselves up. In contrast, `gopls` needs to fully specify the
environment. It can simply pass in the fully evaluated GO* values.

Finally, make the change I was actually here to make: propagate
GOMODCACHE and use it where appropriate.

Fixes golang/go#39761.

Change-Id: I720c69839d91d66d98e94dfc5f065ba0279c5542
Reviewed-on: https://go-review.googlesource.com/c/tools/+/239754
Run-TryBot: Heschi Kreinick <heschi@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Rebecca Stambler <rstambler@golang.org>