cmd/go: modload should use a read-write lock to improve concurrency

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Change-Id: I3f9bc8a2459059a924a04fa02794e258957819b5
GitHub-Last-Rev: 6ad6f6a70e21dc45fc0c880b8732fb87c656c520
GitHub-Pull-Request: golang/go#74311
Reviewed-on: https://go-review.googlesource.com/c/go/+/683215
Reviewed-by: Michael Matloob <matloob@golang.org>
Reviewed-by: Ian Alexander <jitsu@google.com>
Reviewed-by: Michael Matloob <matloob@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

src/cmd/go/internal/modload/init.go

@@ -149,7 +149,7 @@ type MainModuleSet struct {
 	// highest replaced version of each module path; empty string for wildcard-only replacements
 	highestReplaced map[string]string
 
-	indexMu sync.Mutex
+	indexMu sync.RWMutex
 	indices map[module.Version]*modFileIndex
 }
 
@@ -228,8 +228,8 @@ func (mms *MainModuleSet) GetSingleIndexOrNil() *modFileIndex {
 }
 
 func (mms *MainModuleSet) Index(m module.Version) *modFileIndex {
-	mms.indexMu.Lock()
-	defer mms.indexMu.Unlock()
+	mms.indexMu.RLock()
+	defer mms.indexMu.RUnlock()
 	return mms.indices[m]
 }