cmd/go: skip over all workspace modules in go mod verify

This was a remaining place where we made the assumption that there is
only one workspace module. So we'd only skip the first workspace
module when running go mod verify. Instead skip over the first
MainModules.Len() modules of the buildlist, which are all the main
modules.

Fixes #54372

Change-Id: Ife687c907ae4326759c43cc35f78d429d5113b19
Reviewed-on: https://go-review.googlesource.com/c/go/+/485475
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Michael Matloob <matloob@golang.org>
Run-TryBot: Michael Matloob <matloob@golang.org>
Reviewed-by: Bryan Mills <bcmills@google.com>

src/cmd/go/internal/modcmd/verify.go

@@ -58,7 +58,7 @@ func runVerify(ctx context.Context, cmd *base.Command, args []string) {
 
 	// Use a slice of result channels, so that the output is deterministic.
 	const defaultGoVersion = ""
-	mods := modload.LoadModGraph(ctx, defaultGoVersion).BuildList()[1:]
+	mods := modload.LoadModGraph(ctx, defaultGoVersion).BuildList()[modload.MainModules.Len():]
 	errsChans := make([]<-chan []error, len(mods))
 
 	for i, mod := range mods {

src/cmd/go/internal/modload/buildlist.go

@@ -448,7 +448,7 @@ func (mg *ModuleGraph) WalkBreadthFirst(f func(m module.Version)) {
 }
 
 // BuildList returns the selected versions of all modules present in the graph,
-// beginning with Target.
+// beginning with the main modules.
 //
 // The order of the remaining elements in the list is deterministic
 // but arbitrary.

src/cmd/go/testdata/script/work_issue54372.txt

@@ -0,0 +1,37 @@
+# go mod verify should not try to verify the workspace modules.
+# This is a test for #54372.
+
+go mod verify
+stdout 'all modules verified'
+! stderr .
+
+-- go.work --
+go 1.21
+
+use (
+    ./a
+    ./b
+    ./c
+    ./d
+)
+-- a/go.mod --
+module example.com/a
+
+go 1.21
+
+require rsc.io/quote v1.1.0
+-- a/a.go --
+package a
+import _ "rsc.io/quote"
+-- b/go.mod --
+module example.com/b
+
+go 1.21
+-- c/go.mod --
+module example.com/c
+
+go 1.21
+-- d/go.mod --
+module example.com/d
+
+go 1.21