cmd/go/internal/modget: synchronize writes to modOnly map in runGet

Adds an additional lock around an access to modOnly.

Updates #35317

Change-Id: Ia1e75f9a674ec2a2c0489b41283c1cd3e7924d1e
Reviewed-on: https://go-review.googlesource.com/c/go/+/209237
Run-TryBot: Jay Conrod <jayconrod@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>

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

@@ -458,7 +458,9 @@ func runGet(cmd *base.Command, args []string) {
 	modOnly := make(map[string]*query)
 	for _, q := range queries {
 		if q.m.Version == "none" {
+			modOnlyMu.Lock()
 			modOnly[q.m.Path] = q
+			modOnlyMu.Unlock()
 			continue
 		}
 		if q.path == q.m.Path {

src/cmd/go/testdata/script/mod_issue35317.txt

@@ -5,4 +5,4 @@ env GO111MODULE=on
 [short] skip
 
 go mod init example.com
-go get golang.org/x/text@v0.3.0 golang.org/x/internal@v0.1.0
+go get golang.org/x/text@v0.3.0 golang.org/x/internal@v0.1.0 golang.org/x/exp@none