cmd/go: in 'go get', only load retractions for resolved versions

Previously, 'go get' loaded retractions for every module in the build
list, which took a long time and usually wasn't helpful.

This rolls forward CL 269019, which was reverted in CL 270521. The new
revision adds a call to modload.ListModules at the end of 'go get' to
ensure .info files are cached for everything in the build list.

Fixes #42185

Change-Id: I684f66c5e674384d5a0176fbc8317e5530b8a915
Reviewed-on: https://go-review.googlesource.com/c/go/+/270858


Trust: Jay Conrod <jayconrod@google.com>
Reviewed-by: Bryan C. Mills <bcmills@google.com>