tidy: features.rs: collect_lib_features: Simplify
tidy: features.rs: collect_lib_features: Simplify

Use `if let` to simplify a match, and use `contains_key` instead of
`get`.

GBGamer/rust,graydon/rust,graydon/rust,graydon/rust,aidancully/rust,GBGamer/rust,graydon/rust,GBGamer/rust,graydon/rust,aidancully/rust,aidancully/rust,GBGamer/rust,GBGamer/rust,aidancully/rust,GBGamer/rust,aidancully/rust,GBGamer/rust,GBGamer/rust,aidancully/rust,graydon/rust