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

tests/libtest/CMakeLists.txt

@@ -38,7 +38,7 @@ function(setup_test _test_name)  # ARGN are the files in the test
     "${CURL_SOURCE_DIR}/tests/libtest"  # to be able to build generated tests
   )
   if(USE_ARES)
-    include_directories(${CARES_INCLUDE_DIR})
+    include_directories(${CARES_INCLUDE_DIRS})
   endif()
 
   target_link_libraries(${_test_name} ${LIB_SELECTED} ${CURL_LIBS})

tests/server/CMakeLists.txt

@@ -31,7 +31,7 @@ function(setup_executable _test_name)  # ARGN are the files in the test
     "${CURL_SOURCE_DIR}/src"  # for "tool_xattr.h" in disabled_SOURCES
   )
   if(USE_ARES)
-    include_directories(${CARES_INCLUDE_DIR})
+    include_directories(${CARES_INCLUDE_DIRS})
   endif()
 
   target_link_libraries(${_test_name} ${CURL_LIBS})