cmake: flatten build tree, tidy up base dir variables

[vszakats]

• drop generated subdir, move these files to build root.
  To move them next to CPack and other config files, and to avoid
  a subdirectory for only 2 files.
  Follow-up to 6932849 CMake: Improve config installation #2849

• add 'Consumed variables' comment for CMake/cmake_uninstall.in.cmake.

• move generated cmake_uninstall.cmake to the build root directory
  (from CMake/). To:

  • avoid creating a CMake subdirectory within the build directory
    with this single file in it.
  • move it next to its cmake_install.cmake counterpart.
  • move it next to install_manifest.txt which it relies on.

  Follow-up to 27e2a47

• Use PROJECT_SOURCE_DIR for these files, replacing
  CMAKE_CURRENT_SOURCE_DIR, to make it consistent with rest of CMake
  sources, and to reduce ambiguity in CMake/cmake_uninstall.in.cmake
  template.

Follow-up to 8198e38 #22188
Follow-up to 4839029 #22187
Follow-up to 9126eb5 #15331

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Note: the ninja curl_uninstall feature does not work in certain cases.
It also does not remove empty directories.