Add Snippet support with SwiftBuild

[bkhouri]

Snippets are treated as executable targets with the native build system.

This change updates the PIF Builder to support snippet, giving Snippet support with the Swift Build build system.

Depends on: swiftlang/swift-build#775
Fixes: #9040
issue: rdar://158630024
issue: rdar://147705448

[bkhouri]

@swift-ci test

[owenv]

What's here looks good, I just have a few follow up questions

The proposal says:

Snippet targets automatically depend on any library targets declared in their host package, so snippets are free to import those modules.
Are those dependencies getting captured in the PIF right now? Would be good to have a test that imports of API from the package works

It also says:

Snippet targets will be built by default when running swift build --build-snippets. This is consistent with how building tests is an explicit choice, like when running swift test or swift build --build-tests.

Right now, PIFBuilder.buildAggregateProject is adding snippets to the "all-including-tests" target AFAICT, which will cause the snippets to build with just swift test. We might need to add a new aggregate which pulls in snippets that clients can opt into