We need parts of tools/common/SdkVersions.cs when building tests on Windows.
In order to simplify our Windows-life, we're going to check in the generated
SdkVersions.cs file, that way it won't have to be re-generated on Windows (the
logic is very make-based, and not easily executed on Windows).
However, parts of SdkVersions.cs would change every commit, which would make
the above solution rather annoying. So split out those parts into a new file
(ProductConstants.cs), which is still generated during the build (and not
checked in).
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