Add an "Exists" function to fsutil

Exists returns true if a file or directory exist at the given path and
false if one doesn't.

fsutil/fsutil.go

@@ -28,6 +28,20 @@ func BaseNoExt(path string) string {
 	return base
 }
 
+// Exists returns true if a file or directory exist at path and false if one
+// doesn't. If a filesystem error occurs doing the check then Exists returns
+// false and the error.
+func Exists(path string) (bool, error) {
+	_, err := os.Stat(path)
+	if err == nil {
+		return true, nil
+	}
+	if os.IsNotExist(err) {
+		return false, nil
+	}
+	return false, err
+}
+
 // Move moves a file or directory. It first tries to rename src to dst. If the
 // rename fails due to the source and destination being on different file
 // systems Move copies src to dst, then deletes src.

fsutil/fsutil_test.go

@@ -80,6 +80,40 @@ func TestBaseNoExt(t *testing.T) {
 	}
 }
 
+func TestExists(t *testing.T) {
+	t.Parallel()
+
+	td := tfs.NewDir(t, "preprocessing-sfa-test",
+		tfs.WithDir("a", tfs.WithFile("needle", "")),
+	)
+
+	type test struct {
+		name string
+		path string
+		want bool
+	}
+	for _, tt := range []test{
+		{
+			name: "file exists",
+			path: td.Join("a", "needle"),
+			want: true,
+		},
+		{
+			name: "file doesn't exist",
+			path: td.Join("b"),
+			want: false,
+		},
+	} {
+		t.Run(tt.name, func(t *testing.T) {
+			t.Parallel()
+
+			got, err := fsutil.Exists(tt.path)
+			assert.NilError(t, err)
+			assert.Equal(t, got, tt.want)
+		})
+	}
+}
+
 func TestSetFileModes(t *testing.T) {
 	t.Parallel()