build: add missing import.cmake

[anonrig]

When BUILD_TESTING flag is disabled, build crashes at the moment. This is due to the missing import.cmake statement inside tools folder.

An example error from the homebrew pull request:

Ref: https://github.com/Homebrew/homebrew-core/actions/runs/4737212538/jobs/8409814274?pr=128710

Logs

==> cmake -B build -DCMAKE_INSTALL_PREFIX=/opt/homebrew/Cellar/ada/2.1.0 -DCMAKE_INSTALL_LIBDIR=lib -DCMAKE_BUILD_TYPE=Release -DCMAKE_FIND_FRAMEWORK=LAST -DCMAKE_VERBOSE_MAKEFILE=ON -Wno-dev -DBUILD_TESTING=OFF -DCMAKE_OSX_SYSROOT=/Library/Developer/CommandLineTools/SDKs/MacOSX11.sdk
-- The C compiler identification is AppleClang 13.0.0.13000029
-- The CXX compiler identification is AppleClang 13.0.0.13000029
-- Detecting C compiler ABI info
-- Detecting C compiler ABI info - done
-- Check for working C compiler: /opt/homebrew/Library/Homebrew/shims/mac/super/clang - skipped
-- Detecting C compile features
-- Detecting C compile features - done
-- Detecting CXX compiler ABI info
-- Detecting CXX compiler ABI info - done
-- Check for working CXX compiler: /opt/homebrew/Library/Homebrew/shims/mac/super/clang++ - skipped
-- Detecting CXX compile features
-- Detecting CXX compile features - done
-- Code formatting is not enabled.
-- Compiler ID : AppleClang
-- CMAKE_BUILD_TYPE : Release
-- Assuming GCC-like compiler.
-- Found Python3: /opt/homebrew/bin/python3.11 (found version "3.11.3") found components: Interpreter 
-- Python found, we are going to amalgamate.py.
CMake Error at tools/CMakeLists.txt:5 (import_dependency):
  Unknown CMake command "import_dependency".

[anonrig]

Easiest way to reproduce cmake -B build -G Ninja -DBUILD_TESTING=OFF