Support ignoring Packages.swift files

[tristanlabelle]

The vscode-swiftformat extension is causing SPM invocations due to this code, in our repo that is using CMake as the build system on Windows, but still has Packages.swift files for the macOS build. This causes unexpected .build folders to appear. We'd like to always use the globally installed swiftformat and not rely on the SPM package.

[tristanlabelle]

As discussed offline, this could be replacing swiftformat.onlyEnableOnSwiftPMProjects with an enum such as swiftformat.executableSource that could be set to one of auto, machineInstalled, swiftPM and swiftPM-prebuilt.

[tristanlabelle]

We should ignore the old setting if the new one is set and if possible mark the old one as deprecated.