wrong mapping of "datadir" variable

Hi meson-python team,

I encountered this issue when working on https://github.com/dipy/dipy/pull/2715.

With the help of @eli-schwartz, we saw that `get_option('datadir')` does not return the `share` folder. `Meson-python` maps "datadir" to /usr instead of /usr/share -- or more generally, to the python prefix instead of python prefix + "/share".

Thank you in advance for the future fix and let me know if my issue is not clear. Thanks!

Start by reproducing the `get_option('datadir')` behavior described in the issue, using the linked DIPY pull request as context. Trace how meson-python maps `datadir` to the installation prefix; done means it resolves to the prefix plus `/share`, rather than the prefix alone, with coverage for the corrected mapping.

Written by the indexing model from the issue text.