Restore support for explicitly referenced CMakeLists.txt sources

Since commit v3.11.0-rc1~467^2 (VS,Xcode: Add CMakeLists.txt sources
without mutating targets, 2017-10-18) we do not add `CMakeLists.txt` to
target sources but instead generate references to them directly.  This
broke projects that explicitly specify their `CMakeLists.txt` file as a
source file because the explicit entry is no longer consolidated with
the generated one.

Teach the relevant generators to avoid duplicating `CMakeLists.txt`
source references and add test cases.

Fixes: #17828

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