Refactor `next-font-manifest-plugin` (#51835)
## Background
Currently we're track all imported CSS modules by an entry, in the
client manifest:
```js
// client manifest
{
entryCSSFiles: {
'app/entry_name': {
modules: ['app/foo.css', ...]
}
}
}
```
And then, in the font manifest we track all emitted assets (fonts) by
each CSS module:
```js
// font manifest
{
app: {
'app/foo.css': [
'static/font/abc.woff', ...
]
}
}
```
These two fields are only used together by `get-preloadable-fonts.tsx`,
so it can know which font files need to be preloaded for this entry.
(Although previously we use `.modules` for something else, but it's gone
now)
## Changes
Since we only need the font assets per entry, it's unnecessary to track
these in the module level and then join them together. This PR removes
the `modules` field from the client manifest, and changes the font
manifest to directly keep the entry—font mapping.
This gets rid of one module traversal from the client manifest plugin.
