hugofs: Make FileMeta a struct

This commit started out investigating a `concurrent map read write` issue, ending by replacing the map with a struct.

This is easier to reason about, and it's more effective:

```
name                                  old time/op    new time/op    delta
SiteNew/Regular_Deep_content_tree-16    71.5ms ± 3%    69.4ms ± 5%    ~     (p=0.200 n=4+4)

name                                  old alloc/op   new alloc/op   delta
SiteNew/Regular_Deep_content_tree-16    29.7MB ± 0%    27.9MB ± 0%  -5.82%  (p=0.029 n=4+4)

name                                  old allocs/op  new allocs/op  delta
SiteNew/Regular_Deep_content_tree-16      313k ± 0%      303k ± 0%  -3.35%  (p=0.029 n=4+4)
```

See #8749

hugofs/fileinfo_test.go

@@ -0,0 +1,51 @@
+// Copyright 2021 The Hugo Authors. All rights reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package hugofs
+
+import (
+	"testing"
+
+	qt "github.com/frankban/quicktest"
+)
+
+func TestFileMeta(t *testing.T) {
+	c := qt.New(t)
+
+	c.Run("Merge", func(c *qt.C) {
+		src := &FileMeta{
+			Filename: "fs1",
+			Path:     "ps1",
+		}
+		dst := &FileMeta{
+			Filename: "fd1",
+		}
+
+		dst.Merge(src)
+
+		c.Assert(dst.Path, qt.Equals, "ps1")
+		c.Assert(dst.Filename, qt.Equals, "fd1")
+	})
+
+	c.Run("Copy", func(c *qt.C) {
+		src := &FileMeta{
+			Filename: "fs1",
+			Path:     "ps1",
+		}
+		dst := src.Copy()
+
+		c.Assert(dst, qt.Not(qt.Equals), src)
+		c.Assert(dst, qt.DeepEquals, src)
+	})
+
+}