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.

rustyhorde/cargo,nipunn1313/cargo,nipunn1313/cargo,tshepang/cargo,tshepang/cargo,eminence/cargo,tsurai/cargo,rustyhorde/cargo,eminence/cargo,rustyhorde/cargo,tshepang/cargo,rust-lang/cargo,rust-lang/cargo,tsurai/cargo,vojtechkral/cargo,rustyhorde/cargo,rust-lang/cargo,tshepang/cargo,tsurai/cargo,eminence/cargo,eminence/cargo,tsurai/cargo,nipunn1313/cargo,tsurai/cargo,vojtechkral/cargo,vojtechkral/cargo,vojtechkral/cargo,rust-lang/cargo,nipunn1313/cargo