Turn `Workspace::is_member` into an `O(1)` query

Turn `Workspace::is_member` into an `O(1)` query

This commit moves it from a linear query which does a lot of hashing of
`PathBuf` to an `O(1)` query that only hashes `PackageId`. This improves
Servo's null build performance by 50ms or so, causing a number of
functions to disappear from profiles.