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

1 files changed, 4 insertions, 4 deletions

diff --git a/modules/npm/package_builder.go b/modules/npm/package_builder.go
index a073b3d8f..aa9ecb35b 100644
--- a/modules/npm/package_builder.go
+++ b/modules/npm/package_builder.go
@@ -80,12 +80,12 @@ func Pack(fs afero.Fs, fis []hugofs.FileMetaInfo) error {
 	}
 
 	meta := fi.(hugofs.FileMetaInfo).Meta()
-	masterFilename := meta.Filename()
+	masterFilename := meta.Filename
 	f, err := meta.Open()
 	if err != nil {
 		return errors.Wrap(err, "npm pack: failed to open package file")
 	}
-	b = newPackageBuilder(meta.Module(), f)
+	b = newPackageBuilder(meta.Module, f)
 	f.Close()
 
 	for _, fi := range fis {
@@ -100,7 +100,7 @@ func Pack(fs afero.Fs, fis []hugofs.FileMetaInfo) error {
 
 		meta := fi.(hugofs.FileMetaInfo).Meta()
 
-		if meta.Filename() == masterFilename {
+		if meta.Filename == masterFilename {
 			continue
 		}
 
@@ -108,7 +108,7 @@ func Pack(fs afero.Fs, fis []hugofs.FileMetaInfo) error {
 		if err != nil {
 			return errors.Wrap(err, "npm pack: failed to open package file")
 		}
-		b.Add(meta.Module(), f)
+		b.Add(meta.Module, f)
 		f.Close()
 	}