feat: option to preserve other component versions in splat directory

[superewald]

This PR adds an option to preserve SDK/CRT/VCR with different versions in a splat directory which allows to build a shared root folder containing multiple component versions.

Why

As a library consumer, I want to build a single cache folder that contains multiple versions of SDK/CRT/VCR. Currently, splat removes the whole root output directory which would also remove other component versions.

Changes

Add --preserve-versions flag to splat options which forces xwin to only remove the specific version from the splat directory instead of the whole root (eg splat/VCR/14.29.30157 instead of splat/VCR). Then a splat root can contain any number of component versions.

Note

--preserve-versions only works together with --use-winsysroot-style. For the default layout this flag is meaningless (splat still removes the whole root dir).

splat/
    VC/Tools/MSVC/
        14.29.16.10/
        14.30.16.10/
        .../
    VCR/
        14.29.30157/
        14.40.33807/
        .../
    Windows Kits/10/{Include,Lib}/
        10.0.19041/
        10.0.22621/
        .../