Cherry-picks to release/6.0 for sourcekit-lsp update

[ahoppen]

• Explanation: Cherry-pick the following to PRs which are companions of Merge main into release/6.0 sourcekit-lsp#1213
  • Cleanup addition of swift-syntax #7505
  • Report whether a target is part of the root package in the sourcekit-lsp API #7492
• Scope: CMake build and SourceKit-LSP API
• Risk: Low, the SourceKit-LSP API is only used by SourceKit-LSP and the CMake build changes will all be caught at build-time
• Testing: n/a
• Issue: n/a
• Reviewer: @DougGregor and @MaxDesiatov on Cleanup addition of swift-syntax #7505 and Report whether a target is part of the root package in the sourcekit-lsp API #7492

[ahoppen]

swiftlang/swift-syntax#2637
swiftlang/sourcekit-lsp#1213

@swift-ci Please test

[MaxDesiatov]

@swift-ci test windows

[MaxDesiatov]

Windows failure is:

-- Installing: T:/Program Files/Swift/Toolchains/0.0.0+Asserts/usr/lib/swift/host/SwiftIDEUtils.swiftmodule/x86_64-unknown-windows-msvc.swiftinterface
CMake Error at _deps/swiftsyntax-build/Sources/SwiftCompilerPlugin/cmake_install.cmake:45 (file):
  file INSTALL cannot find "T:/12/bin/SwiftCompilerPlugin.dll": No such file
  or directory.
Call Stack (most recent call first):
  _deps/swiftsyntax-build/Sources/cmake_install.cmake:107 (include)
  _deps/swiftsyntax-build/cmake_install.cmake:42 (include)
  BuildSupport/SwiftSyntax/cmake_install.cmake:42 (include)
  cmake_install.cmake:42 (include)

[ahoppen]

swiftlang/swift#73392
swiftlang/swift-syntax#2637
swiftlang/sourcekit-lsp#1213

@swift-ci Please test