cmake: sync up result variable names in Find modules

- bearssl, c-ares, gss, libpsl, libssh2, mbedtls:
  Before this patch these Find modules returned results via
  `<NAME>_INCLUDE_DIR` and `<NAME>_LIBRARY`.

  This patch makes them return `<NAME>_INCLUDE_DIRS` (note the `S`)
  and `<NAME>_LIBRARIES` like other modules already did.

- bearssl, mbedtls:
  Before this patch these Find modules allowed custom configuration
  via `<NAME>_INCLUDE_DIRS` (note the `S`).

  This patch makes them accept `<NAME>_INCLUDE_DIR`, like the rest of
  the modules did.

  Deprecate the old variables, but keep accepting them for
  compatibility.

- bearssl: add missing `mark_as_advanced()` call.

Closes #14542

lib/CMakeLists.txt

@@ -43,7 +43,7 @@ include_directories(
   "${CMAKE_CURRENT_SOURCE_DIR}"
 )
 if(USE_ARES)
-  include_directories(${CARES_INCLUDE_DIR})
+  include_directories(${CARES_INCLUDE_DIRS})
 endif()
 
 if(BUILD_TESTING)