cmd/go: split out cmd/go/internal/load

This is one CL in a long sequence of changes to break up the
go command from one package into a plausible group of packages.

This sequence is concerned only with moving code, not changing
or cleaning up code. There will still be more cleanup after this sequence.

The entire sequence will be submitted together: it is not a goal
for the tree to build at every step.

For #18653.

Change-Id: Ic802483e50598def638f1e2e706d5fdf7822d32d
Reviewed-on: https://go-review.googlesource.com/36196


Reviewed-by: David Crawshaw <crawshaw@golang.org>