internal/mod/modpkgload: skip fs.Stat before fs.ReadDir

Calling fs.Stat before fs.ReadDir used to be crucial to catch cases
where a local package being loaded was a regular file rather than a
directory, and we have had test cases for that scenario for some time.

Since the recent cue/load and modules refactors, this extra io/fs work
appears to no longer be necessary, as we just capture missing files,
which can be done with fs.ReadDir easily.

This reduces the number of syscalls for a single and all packages from

    $ strace -f -t -e trace=file cue fmt ./internal/ci |& grep '/home/mvdan/src' | wc -l
    96
    $ strace -f -t -e trace=file cue fmt ./... |& grep '/home/mvdan/src' | wc -l
    5256

by about 5% and 0.5% to

    $ strace -f -t -e trace=file cue fmt ./internal/ci |& grep '/home/mvdan/src' | wc -l
    90
    $ strace -f -t -e trace=file cue fmt ./... |& grep '/home/mvdan/src' | wc -l
    5224

Updates #3155.

Signed-off-by: Daniel Martí <mvdan@mvdan.cc>
Change-Id: Id706a0ef83e7bf29869c8a99faeb7ae5db1537e9
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/1195413
Unity-Result: CUE porcuepine <cue.porcuepine@gmail.com>
Reviewed-by: Paul Jolly <paul@myitcv.io>
TryBot-Result: CUEcueckoo <cueckoo@cuelang.org>

internal/mod/modpkgload/import.go

@@ -237,22 +237,15 @@ func (pkgs *Packages) findLocalPackage(pkgPath string) ([]module.SourceLoc, erro
 }
 
 func isDirWithCUEFiles(loc module.SourceLoc) (bool, error) {
-	// It would be nice if we could inspect the error returned from
-	// ReadDir to see if it's failing because it's not a directory,
-	// but unfortunately that doesn't seem to be something defined
-	// by the Go fs interface.
-	fi, err := fs.Stat(loc.FS, loc.Dir)
-	if err != nil {
-		if !errors.Is(err, fs.ErrNotExist) {
-			return false, err
-		}
-		return false, nil
-	}
-	if !fi.IsDir() {
-		return false, nil
-	}
+	// It would be nice if we could inspect the error returned from ReadDir to see
+	// if it's failing because it's not a directory, but unfortunately that doesn't
+	// seem to be something defined by the Go fs interface.
+	// For now, catching fs.ErrNotExist seems to be enough.
 	entries, err := fs.ReadDir(loc.FS, loc.Dir)
 	if err != nil {
+		if errors.Is(err, fs.ErrNotExist) {
+			return false, nil
+		}
 		return false, err
 	}
 	for _, e := range entries {