Looking at the code as a whole, we ended up with a little to much "buttons". It turns out that doing case insensitive matching (lower both pattern and strings to match) performs just fine. Or at least, it
gives the penalty to the people who uses mixed case filenames.
```
GetGlob/Default_cache-10 10.6ns ± 2% 10.6ns ± 1% ~ (p=0.657 n=4+4)
GetGlob/Filenames_cache,_lowercase_searchs-10 10.6ns ± 2% 10.6ns ± 0% ~ (p=1.000 n=4+4)
GetGlob/Filenames_cache,_mixed_case_searchs-10 29.7ns ± 1% 29.6ns ± 1% ~ (p=0.886 n=4+4)
GetGlob/GetGlob-10 13.7ns ± 1% 13.7ns ± 0% ~ (p=0.429 n=4+4)
name old alloc/op new alloc/op delta
GetGlob/Default_cache-10 0.00B 0.00B ~ (all equal)
GetGlob/Filenames_cache,_lowercase_searchs-10 0.00B 0.00B ~ (all equal)
GetGlob/Filenames_cache,_mixed_case_searchs-10 5.00B ± 0% 5.00B ± 0% ~ (all equal)
GetGlob/GetGlob-10 0.00B 0.00B ~ (all equal)
name old allocs/op new allocs/op delta
GetGlob/Default_cache-10 0.00 0.00 ~ (all equal)
GetGlob/Filenames_cache,_lowercase_searchs-10 0.00 0.00 ~ (all equal)
GetGlob/Filenames_cache,_mixed_case_searchs-10 1.00 ± 0% 1.00 ± 0% ~ (all equal)
GetGlob/GetGlob-10
```
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
