Watch mode optimizations

- No need to re-analyze or re-create watchers unless a package.json file has changed (and watchers only need to be re-created if a `files` array changed, and even then only the affected ones do).

- We can share the `CachingPackageJsonReader` instance across analysis. File change events tell us when a cached result is stale.

- We can cache glob results in memory. File change events tell us when a cached result is stale.

- We can cache file hashes in memory. File change events tell us when a cached result is stale.