Rather than injecting `CMakeLists.txt` files into each target's
`SOURCES`, teach the generators to add them during generation using
dedicated code.  This avoids mutating the original targets, and avoids
polluting `$<TARGET_PROPERTY:foo,SOURCES>` with generator-specific
content.

This also avoids listing the `CMakeLists.txt` sources in the results of
`CMAKE_DEBUG_TARGET_PROPERTIES==SOURCES` so the `RunCMake.TargetSources`
test no longer needs a separate case for IDEs.