[Traits] Change TraitConfiguration struct to an enum

[bripeticca]

Description

Currently, the TraitConfiguration struct describes a user-designated trait configuration wherein the user can decide which combination of traits are enabled/disabled for a given package. The way this struct is currently designed allows for an optional set of enabled traits, as well as a bool that describes the flag to enableAllTraits. Since there are distinct use cases depending on whether or not the enabled traits are nil, empty, or contains a list of traits, it makes more sense to treat this as an enum:

enum TraitConfiguration: Codable, Hashable { 
    case enableAllTraits // when the enableAllTraits flag is set, since this will simply enable everything
    case noConfiguration // when there isn't a configuration set, meaning that the default traits will be used
    case noTraits // when there is a configuration set and the set of enabled traits is empty, which overrides the default traits 
    case traits(Set<String>) // when there is a configuration set and there is a set of traits present
}

Expected behavior

This will make the checks done on TraitConfiguration to be more ergonomic, as there would be a defined set of cases that are explicitly named and would omit redundant checks.

Actual behavior

The current use of TraitConfiguration can be a bit painful - there are plenty of condition checks involving repeated terms (i.e. if let enabledTraits = traitConfiguration.enabledTraits, traitConfiguration.enableAllTraits == false { ... } and other variations of this) that can be condensed for ease of use.

Steps to reproduce

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Swift Package Manager version/commit hash

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Swift & OS version (output of swift --version && uname -a)

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