cmd/go: do context propagation for tracing downloads
This change does context propagation (and only context propagation)
necessary to add context to modfetch.Download and pkg.LoadImport.
This was done by adding context to their callers, and then
adding context to all call-sites, and then repeating adding
context to callers of those enclosing functions and their
callers until none were left. In some cases the call graph expansion
was pruned by using context.TODOs.
The next CL will add a span to Download. I kept it out of this
change to avoid making it any larger (and harder to review)
than it needs to be.
Updates #38714
Change-Id: I7a03416e04a14ca71636d96f2c1bda2c4c30d348
Reviewed-on: https://go-review.googlesource.com/c/go/+/249021
Run-TryBot: Michael Matloob <matloob@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org> Reviewed-by: Jay Conrod <jayconrod@google.com>