cmd/go: make 'go mod why -m' work in inconsistent, pruned module

'go mod why -m' works by listing modules matching command line
arguments, then loading "all" packages and finding which of the listed
modules provide packages imported by the main module.

If go.mod is inconsistent (that is, a requirement has a lower version
than MVS would select when the module graph is loaded) and pruned
(that is, the module graph is only loaded when necessary), then
modload.ListModules may return modules with different versions than
would be selected in modload.LoadPackages.

'go mod why -m' was too strict about this, mapping module paths and
versions to packages. With this fix, it maps module paths without
versions to packages.

Fixes #48613

Change-Id: I836c46289bb647d6c46ec65e7589531da532d5e8
Reviewed-on: https://go-review.googlesource.com/c/go/+/352115
Trust: Jay Conrod <[email protected]>
Run-TryBot: Jay Conrod <[email protected]>
TryBot-Result: Go Bot <[email protected]>
Reviewed-by: Bryan C. Mills <[email protected]>

Author: Jay Conrod

src/cmd/go/internal/modcmd/why.go

@@ -12,8 +12,6 @@ import (
 	"cmd/go/internal/base"
 	"cmd/go/internal/imports"
 	"cmd/go/internal/modload"
-
-	"golang.org/x/mod/module"
 )
 
 var cmdWhy = &base.Command{
@@ -90,19 +88,19 @@ func runWhy(ctx context.Context, cmd *base.Command, args []string) {
 			base.Fatalf("go: %v", err)
 		}
 
-		byModule := make(map[module.Version][]string)
+		byModule := make(map[string][]string)
 		_, pkgs := modload.LoadPackages(ctx, loadOpts, "all")
 		for _, path := range pkgs {
 			m := modload.PackageModule(path)
 			if m.Path != "" {
-				byModule[m] = append(byModule[m], path)
+				byModule[m.Path] = append(byModule[m.Path], path)
 			}
 		}
 		sep := ""
 		for _, m := range mods {
 			best := ""
 			bestDepth := 1000000000
-			for _, path := range byModule[module.Version{Path: m.Path, Version: m.Version}] {
+			for _, path := range byModule[m.Path] {
 				d := modload.WhyDepth(path)
 				if d > 0 && d < bestDepth {
 					best = path

src/cmd/go/testdata/script/mod_skip_write.txt

@@ -24,9 +24,8 @@ go mod why rsc.io/sampler
 cmp stdout why.want
 cmp go.mod go.mod.edit
 
-# TODO(#48613): 'go mod why -m' incorrectly reports sampler is not needed.
 go mod why -m rsc.io/sampler
-cmp stdout why-broken.want
+cmp stdout why.want
 cmp go.mod go.mod.edit
 
 cp go.mod.orig go.mod
@@ -91,6 +90,3 @@ rsc.io/[email protected] golang.org/x/[email protected]
 m
 rsc.io/quote
 rsc.io/sampler
--- why-broken.want --
-# rsc.io/sampler
-(main module does not need module rsc.io/sampler)