Implement `package.json`'s `imports` field (#4895)
### Description
The [imports field](https://nodejs.org/api/packages.html#imports) is
similar to `exports`. While `exports` allows a package to control how
others can import it, `imports` allows it control how it wants to import
other packages.
The majority of the code can be shared with the already-implemented
`ExportsField` and `AliasMap`, with just special parsing logic. Besides
that, I just needed to setup the conditional for imports and some
cleanup.
### Testing Instructions
```bash
RUST_BACKTRACE=1 cargo nextest run -E 'test(snapshot__imports__subpath_imports)'
```
Fixes WEB-50
Pair: https://github.com/vercel/next.js/pull/49636
fix #4799
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Co-authored-by: Tobias Koppers <tobias.koppers@googlemail.com>
