Introduces a `SyncReadAsyncWriteLock` to make it harder to write to the
npm resolution without first waiting async in a queue. For the npm
resolution, reading synchronously is fine, but when updating, someone
should wait async, clone the data, then write the data at the end back.
This functionality was broken. The series of events was:
1. Load the npm resolution from the lockfile.
2. Discover only a subset of the specifiers in the documents.
3. Clear the npm snapshot.
4. Redo npm resolution with the new specifiers (~500ms).
What this now does:
1. Load the npm resolution from the lockfile.
2. Discover only a subset of the specifiers in the documents and take
into account the specifiers from the lockfile.
3. Do not redo resolution (~1ms).
