[Traits] Package trait description and serialization

[FranzBusch]

Motivation:

This PR is the first one to land the package traits feature in SwiftPM. It focuses on the new public APIs and serialization of them.

Modifications:

This PR contains:

• The PackageDescription APIs used to define traits, optional dependencies based on traits and build settings based on traits
• The serialization logic to encode the trait specific changes in the package description
• The deserialization logic to decode the trait specific changes into the PackageModel
• Validation logic for traits
• Tests for all of the above

I gated the new APIs behind both the 999 availability and an SPI(Traits) to allow us to land this on main before the proposal is accepted.

Result:

We can now define a package with traits 🚀

[FranzBusch]

@swift-ci please test

[FranzBusch]

@swift-ci please test

[FranzBusch]

@swift-ci please test

[FranzBusch]

@swift-ci please test

[FranzBusch]

@swift-ci please test

[MaxDesiatov]

@swift-ci test windows

[MaxDesiatov]

@bnbarham do you think that @_spi is sufficient here, or do we need a separate CLI flag with experimental- prefix to enable it?

[FranzBusch]

@MaxDesiatov @bnbarham This is both gated by a new availability and SPI. So it's really hard to access.

[MaxDesiatov]

@swift-ci test

[bnbarham]

@bnbarham do you think that @_spi is sufficient here, or do we need a separate CLI flag with experimental- prefix to enable it?

LGTM after the ExperimentalTraits SPI change 👍

[FranzBusch]

@swift-ci please test

[FranzBusch]

@MaxDesiatov Fixed up your comments. Thanks for catching the bad merge w.r.t. moduleAlias stuff

[MaxDesiatov]

@swift-ci test windows