cmd/gomobile: emit replace directives without a version

bind builds the bindings in a generated module and reconstructs its
build list from 'go list -m -json all' run in the original module. For
a replaced module it emitted the replacement scoped to the version the
original module selected, and emitted no requirement at all.

The generated module requires a different set of modules than the
original one does, so minimal version selection can select a higher
version: requirements that module graph pruning hides in the original
module become direct requirements of the generated module. The scoped
directive then matches nothing, and the module is fetched from the
module proxy instead of the replacement. Nothing reports an error; the
build just compiles sources other than the ones the original module
resolves to.

Emit replacements without a version so that they stay effective
whichever version is selected, and record the selected version as a
requirement.

Extract the translation into parseModuleVersions so that it can be
tested without running the go command.

Fixes golang/go#80816

Change-Id: Ifd93b8b5ed8b216982a804ff9f4bfdf6aa439209
Reviewed-on: https://go-review.googlesource.com/c/mobile/+/812720
LUCI-TryBot-Result: golang-scoped@luci-project-accounts.iam.gserviceaccount.com <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Mark Freeman <markfreeman@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>