[roblabla]

At $WORK, we try to avoid hitting external services whenever we can, preferring to mirror as much as possible the content we use locally, so that outages don't affect us as much.

We use xwin to generate a cross-compilation toolchain from linux to windows. Unfortunately, this generation step is currently tied to microsoft's servers, and we have had CI failures related to microsoft's servers either being down or refusing to serve certain files from the CDN in the past. To remediate this, we'd like to be able to mirror the parts of the CDN we use locally.

To achieve this, this patch series does two things:

1. It adds a new flag --package-manifest which allows passing the "package manifest" (aka VSMan, aka Catalog) to xwin. xwin already has a --manifest flag, but unfortunately that takes a channel manifest instead of a package manifest, so it still requires "dereferencing" said channel manifest to grab the package manifest.
2. It adds support for a --layout-url flag, which should contain a Visual Studio layout - the official way to create a VS mirror. Those layouts should be created using the visual studio installer's --layout flag, ergo vs_community.exe --layout layout\. The layout can then be uploaded to an HTTP server and passed to other invocations to avoid hitting the MS servers.

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FWIW, there are some open questions here:

1. Instead of adding a new --package-manifest, perhaps the existing --manifest could detect whether the file being passed is a channel or package manifest, and automatically adapt?
2. Instead of using a layout, we may want to use a different mirroring mechanism? I went with layout because that's what VS uses internally, but if a different mirroring solution would be prefered, that's totally OK for me!