[WIP] Fix for required dependencies calculation with traits

[bripeticca]

This PR introduces the EnabledTrait model and associated infrastructure to properly track and manage trait enablement throughout the dependency resolution process. The new system replaces simple string-based trait tracking with a rich model that maintains provenance information about how and why each trait was enabled.

Core Changes

New EnabledTrait Model (EnabledTrait.swift)

Introduces three main components for tracking enabled traits for packages:

EnabledTrait - Represents an individual enabled trait with:

• Trait name (used as the identifier)
• Set of Setter values tracking how the trait was enabled (via command-line config, parent package, or another trait)
• Support for unifying traits with the same name by merging their setter lists - it's important to note that this is usually done with an EnabledTrait for a specific package, and that many packages can define their own (unique) trait that shares a name with a trait from another package but are treated as fundamentally different instances.

EnabledTraits - Collection wrapper around IdentifiableSet<EnabledTrait> that:

• Automatically unifies traits with the same name when inserted
• Provides convenient set operations (union, intersection)
• Maintains enablement metadata while treating traits with the same name as equal (merges Setters for traits of the same name)

EnabledTraitsMap - Thread-safe map storing enabled traits per package that:

• Maps PackageIdentity to EnabledTraits
• Implicitly returns ["default"] for packages with no explicitly enabled traits
• Uses union behaviour: setting traits for the same package multiple times accumulates all traits
• Tracks disablers to distinguish "no explicitly enabled traits" from "default traits explicitly disabled"

The introduction of these models have also provided some API conveniences:

String Interoperability - EnabledTrait and EnabledTraits provide seamless string integration:

• ExpressibleByStringLiteral and ExpressibleByArrayLiteral for natural initialization of enabled traits and sets of enabled traits: let enabledTrait: EnabledTrait = "foo" or let enabledTraits: EnabledTraits = ["foo", "bar"]
• Bidirectional string equality: enabledTrait == "foo" and "foo" == enabledTrait both work
• EnabledTraitConvertible protocol allows APIs to accept string collections and auto-convert them to EnabledTrait

Collection Protocol Support - EnabledTraits conforms to Collection with set-like operations (union, intersection, contains) and overloaded methods that accept either Collection<String> or Collection<EnabledTrait>, enabling flexible APIs that work with both types

Refactored Trait Handling (Manifest+Traits.swift)

• Replaced all Set<String> trait representations with richer EnabledTraits model
• Updated validation methods to work with EnabledTrait instead of raw strings
• Removed parentPackage parameters from validation (now tracked via EnabledTrait.Setter)
• Improved type safety and metadata tracking throughout trait calculation

Workspace+Traits.swift (new file)

• Extracted trait-specific workspace operations
• updateEnabledTraits(for:) method determines enabled traits for loaded manifests
• updateEnabledTraits(forDependency:_:) handles dependency-specific trait propagation

Bug Fixes

Required Dependencies Calculation

Problem: During PubGrub dependency resolution, enabled traits were being reset to defaults instead of propagating previously calculated values when creating DependencyResolutionNode instances, which relied on an accurate set of enabledTraits.

Impact: Wrong trait sets were considered when evaluating required dependencies, leading to incorrect dependency graphs.

Solution: Properly propagate enabled traits through the nodes() method for PackageContainerConstraint, which creates instances of DependencyResolutionNode from these constraints during PubGrub dependency resolution.

IdentifiableSet Intersection

Problem: The intersection method created an empty set instead of starting with self, causing loss of element data.

Solution: Changed var result = Self() to var result = self in IdentifiableSet.swift:86, ensuring proper preservation of element data during intersections.

Test Improvements

• Separated traits tests into dedicated files:
  • ModulesGraphTests+Traits.swift
  • WorkspaceTests+Traits.swift
  • EnabledTraitTests.swift

Additional Improvements

• Enhanced IdentifiableSet with remove(id:) and remove(_:) methods
• Added Workspace+Traits.swift for trait-specific workspace operations
• Updated all package container implementations for proper trait handling

[bripeticca]

@swift-ci please test

[bripeticca]

@swift-ci please test

[bripeticca]

@swift-ci please test

[bripeticca]

@swift-ci please test