Move SwiftSyntaxBuilderGeneration to separate package that pins a version of swift-syntax

[fwcd]

This PR addresses #577 by moving SwiftSyntaxBuilderGeneration to a separate package, i.e. replaces the current package structure

swift-syntax
└─ Sources
   ├─ SwiftSyntaxBuilder
   ├─ SwiftSyntaxBuilderGeneration
   └─ ...

with

swift-syntax
├─ Sources
│  ├─ SwiftSyntaxBuilder
│  └─ ...
└─ SwiftSyntaxGeneration
   └─ Sources
      └─ swift-syntax-builder-generation

The subpackage pins a specific commit of swift-syntax as a dependency and thereby is no longer susceptible to the bootstrapping problems outlined in #577 when adding new syntax nodes.

Things to figure out:

• The build script, in particular how it has to be updated
• How this plays with CI and its workspaces

Therefore I will mark this PR as a draft for now, so we can discuss these things here (and whether it's a good approach to take at all).

cc @ahoppen @CodaFi

[fwcd]

Minor note: The reason for renaming SwiftSyntaxBuilderGeneration to swift-syntax-builder-generation (kebab-case) is twofold:

1. Until the pinned revision of swift-syntax no longer has a target named SwiftSyntaxBuilderGeneration the name will conflict with this target
2. Most Swift repos follow the convention of naming executable targets in kebab-case (e.g. this repo's lit-test-helper, SourceKit-LSP's sourcekit-lsp, SwiftFormat's swift-format, ...). Therefore this would be good opportunity to update the name.

[ahoppen]

I don’t think this is necessary. We just need to make code generation a three-step instead of a two-step process, which I’m doing in #583.

[ahoppen]

I think renaming SwiftSyntaxBuilderGeneration to swift-syntax-builder-generation would still be a change that’s worthwhile.

[ahoppen]

The underlying build issue has been fixed by #583