cmd/go: do not require the module cache to exist for 'go mod edit'

Updates #46695

Change-Id: I4afbc1401ef4183d94c1ac6271394fac1fff95ae
Reviewed-on: https://go-review.googlesource.com/c/go/+/328769
Trust: Bryan C. Mills <bcmills@google.com>
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>

src/cmd/go/internal/modfetch/cache.go

@@ -152,7 +152,7 @@ func lockVersion(mod module.Version) (unlock func(), err error) {
 // If err is nil, the caller MUST eventually call the unlock function.
 func SideLock() (unlock func(), err error) {
  if err := checkCacheDir(); err != nil {
- base.Fatalf("go: %v", err)
+ return nil, err
  }
 
  path := filepath.Join(cfg.GOMODCACHE, "cache", "lock")

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

@@ -0,0 +1,15 @@
+# 'go mod edit' opportunistically locks the side-lock file in the module cache,
+# for compatibility with older versions of the 'go' command.
+# It does not otherwise depend on the module cache, so it should not
+# fail if the module cache directory cannot be created.
+
+[root] skip
+
+mkdir $WORK/readonly
+chmod 0555 $WORK/readonly
+env GOPATH=$WORK/readonly/nonexist
+
+go mod edit -go=1.17
+
+-- go.mod --
+module example.com/m