cmd/go/internal/modload: remove SetBuildList

For the last remaining call site (in cmd/go/internal/work, added for
the new 'go install pkg@version' codepath in CL 254365), use
EditBuildList instead.

SetBuildList assumes that the caller has enough information to produce
a complete, coherent build list. With lazy loading, producing a
complete, coherent build list is no longer quite so trivial.

In CL 263267, I rewrote the main caller of SetBuildList (the 'go get'
command), and in the process added a more targeted modload hook
(EditBuildList). That hook also suffices for 'go install pkg@version'.
The resulting error messages are perhaps not as smooth as they ought
to be, but if they are too awkward we should probably fix them for
'go get' too, and the commands can continue to share the edit hook.

For #36460
Updates #40276

Change-Id: I698a9dcd2efe6378a4d91f21362880aa8e50001b
Reviewed-on: https://go-review.googlesource.com/c/go/+/270980


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