[Traits] Correctly diagnose unused dependency with traits

[FranzBusch]

Motivation

We are currently diagnosing unused dependencies by looking at the list of products used by the final modules and check if for every root dependency we are using at least one product of that package. This incorrectly diagnosis unused dependencies if the usage of the dependency is either:

• Just declared to enable a trait if down the graph another package is depending on it
• Used as an optional dependency and the trait is disabled

Modification

This PR adds two more checks to the diagnostic pass to find unused dependencies. We first create a set of dependency products that are guarded by a trait. When checking each dependency we then check if we enabled any trait on the dependency since this means it is used for setting a trait. We also check if any product of the dependency is guarded by a trait to identify if it was an optional dependency.

Result

No more incorrect unused dependency warnings.

[FranzBusch]

@swift-ci please test

[FranzBusch]

@swift-ci test windows

[MaxDesiatov]

@swift-ci test

[MaxDesiatov]

@swift-ci test windows