cmd/go/internal/mvs: split Reqs into narrower per-function interfaces

Reqs currently combines requirements with upgrades and downgrades.
However, only Upgrade needs the Upgrade method, and only Downgrade
needs the Previous method.

When we eventually add lazy loading, the lazily-loaded module graph
will not be able to compute upgrades and downgrades, so the
implementation work from here to there will be clearer if we are
explicit about which are still needed.

For #36460

Change-Id: I7bf8c2a84ce6bc4ef493a383e3d26850e9a6a6c0
Reviewed-on: https://go-review.googlesource.com/c/go/+/290771
Trust: Bryan C. Mills <bcmills@google.com>
Reviewed-by: Michael Matloob <matloob@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>

src/cmd/go/internal/mvs/mvs.go

@@ -41,6 +41,11 @@ type Reqs interface {
 	// Note that v1 < v2 can be written Max(v1, v2) != v1
 	// and similarly v1 <= v2 can be written Max(v1, v2) == v2.
 	Max(v1, v2 string) string
+}
+
+// An UpgradeReqs is a Reqs that can also identify available upgrades.
+type UpgradeReqs interface {
+	Reqs
 
 	// Upgrade returns the upgraded version of m,
 	// for use during an UpgradeAll operation.
@@ -54,6 +59,11 @@ type Reqs interface {
 	// TODO(rsc): Upgrade must be able to return errors,
 	// but should "no latest version" just return m instead?
 	Upgrade(m module.Version) (module.Version, error)
+}
+
+// A DowngradeReqs is a Reqs that can also identify available downgrades.
+type DowngradeReqs interface {
+	Reqs
 
 	// Previous returns the version of m.Path immediately prior to m.Version,
 	// or "none" if no such version is known.
@@ -323,7 +333,7 @@ func Req(target module.Version, base []string, reqs Reqs) ([]module.Version, err
 
 // UpgradeAll returns a build list for the target module
 // in which every module is upgraded to its latest version.
-func UpgradeAll(target module.Version, reqs Reqs) ([]module.Version, error) {
+func UpgradeAll(target module.Version, reqs UpgradeReqs) ([]module.Version, error) {
 	return buildList(target, reqs, func(m module.Version) (module.Version, error) {
 		if m.Path == target.Path {
 			return target, nil
@@ -335,7 +345,7 @@ func UpgradeAll(target module.Version, reqs Reqs) ([]module.Version, error) {
 
 // Upgrade returns a build list for the target module
 // in which the given additional modules are upgraded.
-func Upgrade(target module.Version, reqs Reqs, upgrade ...module.Version) ([]module.Version, error) {
+func Upgrade(target module.Version, reqs UpgradeReqs, upgrade ...module.Version) ([]module.Version, error) {
 	list, err := reqs.Required(target)
 	if err != nil {
 		return nil, err
@@ -374,7 +384,7 @@ func Upgrade(target module.Version, reqs Reqs, upgrade ...module.Version) ([]mod
 // The versions to be downgraded may be unreachable from reqs.Latest and
 // reqs.Previous, but the methods of reqs must otherwise handle such versions
 // correctly.
-func Downgrade(target module.Version, reqs Reqs, downgrade ...module.Version) ([]module.Version, error) {
+func Downgrade(target module.Version, reqs DowngradeReqs, downgrade ...module.Version) ([]module.Version, error) {
 	// Per https://research.swtch.com/vgo-mvs#algorithm_4:
 	// “To avoid an unnecessary downgrade to E 1.1, we must also add a new
 	// requirement on E 1.2. We can apply Algorithm R to find the minimal set of