Make cython generated files less nested to avoid MAX_PATH issues on Windows

[lesteve]

Fix #15123.

Followed #15123 (comment) which recommends to special-case Cython.

• fixed the code, there are two different code paths
  • .pyx file
  • custom target that generated a .pyx file
• added a test to make sure that the generated .pyx.c is directly under the .p folder (something like foo.cpython-313-x86_64-linux-gnu.so.p on Linux)
• added a pyx.c test cases/cython/2 generated sources/libdir/foo.pyx to test one of the code path

I double-checked it seems to work fine with scikit-learn before our work-arounds to use a generator.

[lesteve]

It looks like adding test cases/cython/2 generated sources/libdir/foo.pyx is making another test fail, I need to take a closer look.

[dcbaker]

fname can have path seperators, it's the path relative to the directory that the File object is defined in, so if the src is files('foo/bar/x.pyx'), then fname will reflect that.

[lesteve]

I think the failing test is because Cython should probably espace spaces in paths inside dependency files, see #15227.

I'll try to think of a simpler way to write the test for this PR.

[lesteve]

I tackled #15219 (review) and the case of having src.fname that contains / is handled in the added test.

I ended up creating a separate test in test cases/cython/5 nested folders which is simpler.

[lesteve]

Now all the GHA CIs pass but two Azure Windows CIs fail, I need to take a closer look:

One of the failure seems to be a ninja error ninja: error: failed recompaction: Permission denied build log

ninja explain: output build.ninja older than most recent input ../test cases/cython/5 nested folders/meson.build (7844182826928296 vs 7844182857252785)
[0/1] "C:\hostedtoolcache\windows\Python\3.7.9\x64\python.exe" "D:\a\1\s\meson.py" "--internal" "regenerate" "D:\a\1\s\test cases\cython\5 nested folders" "."
ninja: error: failed recompaction: Permission denied

The other one is a meson setup error build log

ERROR: Could not guess language from source file package/metrics/metric.pyx.