cmd/go/internal/{modcmd,modload}: lock edits to go.mod

Use an arbitrary lockfile to serialize edits, and use atomic renames
to actually write the go.mod file so that we never drop version
requirements due to a command failing partway through a write.

Multiple invocations of the 'go' command may read the go.mod file
concurrently, and will see some consistent version even if some other
invocation changes it concurrently.

Multiple commands may attempt to write the go.mod file concurrently.
One writer will succeed and write a consistent, complete go.mod file.
The others will detect the changed contents and fail explicitly: it is
not, in general, possible to resolve two conflicting changes to module
requirements, so we surface the problem to the user rather than trying
to solve the problem heuristically.

Updates #26794

Change-Id: Ia1a06a01ef93fa9be664f560eb83bb86b0207443
Reviewed-on: https://go-review.googlesource.com/c/146380


Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>