Respect exports mappings in custom imports resolver

[delanni]

Some packages declare an exports field that prescribes how imported paths should be mapped to real file paths.

This breaks our import resolvers, because we're not respecting any path mapping from there.

As in #215627 for example, we had to create a package-specific mapping for any import to be searched for in the dist folder.

We should devise a bit of generic logic in src/platform/packages/private/kbn-import-resolver/src/import_resolver.ts to parse these exports mappings and instruct the resolver to find the right files.