[dev.cmdgo] cmd/go: sort roots when joining multiple main module roots

When concatenating multiple main modules' roots we need to resort
the list of main module roots. This avoids a panic from unsorted main
module roots. This will get better when we have structured module
roots.

Change-Id: I68fed47b0f3b131ed3dadf45db3c442286a0ced0
Reviewed-on: https://go-review.googlesource.com/c/go/+/338111
Trust: Michael Matloob <matloob@golang.org>
Run-TryBot: Michael Matloob <matloob@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>

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

@@ -1083,6 +1083,9 @@ func updateEagerRoots(ctx context.Context, direct map[string]bool, rs *Requireme
 		}
 		roots = append(roots, min...)
 	}
+	if MainModules.Len() > 1 {
+		module.Sort(roots)
+	}
 	if rs.depth == eager && reflect.DeepEqual(roots, rs.rootModules) && reflect.DeepEqual(direct, rs.direct) {
 		// The root set is unchanged and rs was already eager, so keep rs to
 		// preserve its cached ModuleGraph (if any).

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

@@ -36,6 +36,11 @@ cp go.work.backup go.work
 cp go.work.d go.work
 go run example.com/d
 
+# Test that we don't run into "newRequirements called with unsorted roots"
+# panic with unsorted main modules.
+cp go.work.backwards go.work
+go run example.com/d
+
 -- go.work.dup --
 go 1.17
 
@@ -109,3 +114,12 @@ import "example.com/b/lib"
 func main() {
 	lib.Hello()
 }
+
+-- go.work.backwards --
+go 1.18
+
+directory (
+    d
+    b
+    a
+)