On Linux, `hugofs.Glob` does not hit any directories which includes
uppercase letters. (This does not happen on macOS.)
Since `resources.GetMatch/Match` uses `Glob`,
```
{{ resources.GetMatch "Foo/bar.css" }}
```
this does not match `assets/Foo/bar.css` .
On the other hand, you can get it with
```
{{ resources.Get "Foo/bar.css" }}
```
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
