This matches the tests for Python extensions.
Also include some other cleanups to these `meson.build` files:
Adding `python_dep` is no longer needed, this is automatic now.
Use a single line for `import('python').find_installation()`,
because the result of `import('python')` by itself is not used
for anything.
This is a follow-up to gh-8706, which contained the initial fix
to ninjabackend.py but somehow lost it. This re-applies the fix
and adds a test for it.
Without the fix, the error is:
ninja: error: 'ct2.pyx', needed by 'libdir/ct2.cpython-39-x86_64-linux-gnu.so.p/ct2.pyx.c',
missing and no known rule to make it
Ralf Gommers