cmd/go/internal/modget: fix get pkg@none

Now pkg@none actually removes the pkg instead of dying.

For #26342.

Change-Id: I9df7281ed8fd24480109b36f33a563f92e279244
Reviewed-on: https://go-review.googlesource.com/124796
Reviewed-by: Bryan C. Mills <bcmills@google.com>

src/cmd/go/internal/modget/get.go

@@ -356,16 +356,17 @@ func runGet(cmd *base.Command, args []string) {
 	}
 	lookup.Do(10, func(item interface{}) {
 		t := item.(*task)
+		if t.vers == "none" {
+			// Wait for downgrade step.
+			t.m = module.Version{Path: t.path, Version: "none"}
+			return
+		}
 		m, err := getQuery(t.path, t.vers, t.forceModulePath)
 		if err != nil {
 			base.Errorf("go get %v: %v", t.arg, err)
 			return
 		}
 		t.m = m
-		if t.vers == "none" {
-			// Wait for downgrade step.
-			return
-		}
 		// If there is no -u, then we don't need to upgrade the
 		// collected requirements separately from the overall
 		// recalculation of the build list (modload.ReloadBuildList below),

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

@@ -1,8 +1,15 @@
 env GO111MODULE=on
 
-go get rsc.io/quote@v2.0.0
+# downgrade sampler should downgrade quote
+go get rsc.io/sampler@v1.0.0
 go list -m all
-stdout 'rsc.io/quote v0.0.0-'
+stdout 'rsc.io/quote v1.4.0'
+stdout 'rsc.io/sampler v1.0.0'
+
+# downgrade sampler away should downgrade quote further
+go get rsc.io/sampler@none
+go list -m all
+stdout 'rsc.io/quote v1.3.0'
 
 -- go.mod --
 module x