Warning for leaked "private" variables

MSBuild doesn't currently have any scoping concept for properties or items (adding one is #4074). As a stopgap, we could apply a heuristic, opt-in warning that some patterns are likely to be bad, with a suggested fix.

One possibility would be:

* If an item name is prefixed with an underscore, and
* It is not referenced in any target other than the one that defines it,

warn that it should be emptied before returning.

The goal here would be to reduce SDK authoring errors like dotnet/sdk#3027 where an item name gets inadvertently reused.