cmd/go: flip relationship between load and modload

Previously, modload imported load, but it mainly just did so in order
to install callbacks to the modload API. This was important during vgo
development, but there's no longer a strong reason to do this. Nothing
modload imports strongly depends on load, so there's little danger of
a dependency cycle.

This change deletes the callbacks in load and instead, makes load call
exported functions in modload directly. In the future, these functions
may have different signatures than their GOPATH counterparts.

Change-Id: Ifde5c3ffebd190b5bd184924ec447d272b936f27
Reviewed-on: https://go-review.googlesource.com/c/go/+/255719


Trust: Jay Conrod <jayconrod@google.com>
Run-TryBot: Jay Conrod <jayconrod@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>