Improve hint for installing meson-python when missing as build backend (#15826)

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## Summary

If a package uses meson-python as backend, it's declared as:
```
[build-system]
build-backend = 'mesonpy'
requires = ['meson-python']
```

Currently, if meson-python is missing, one gets the following hint:
```
Traceback (most recent call last):
        File "<string>", line 8, in <module>
          import mesonpy as backend
      ModuleNotFoundError: No module named 'mesonpy'

      hint: This error likely indicates that `sagemath` depends on `mesonpy`, but doesn't declare it as a build
      dependency. If `sagemath` is a first-party package, consider adding `mesonpy` to its `build-system.requires`.
      Otherwise, either add it to your `pyproject.toml` under:

      [tool.uv.extra-build-dependencies]
      sagemath = ["mesonpy"]

      or `uv pip install mesonpy` into the environment and re-run with `--no-build-isolation`.
```

which is not quite correct as the build backend/module is called
"mesonpy" but the python package one has to install is "meson-python".
This hint is improved in this PR.


<!-- What's the purpose of the change? What does it do, and why? -->

## Test Plan

I didn't yet had a chance to setup a local dev env for testing this.
<!-- How was it tested? -->

crates/uv-build-frontend/src/pipreqs/mapping

@@ -777,6 +777,7 @@ marks:pytest_marks
 markupsafe:MarkupSafe
 mavnative:pymavlink
 memcache:python_memcached
+mesonpy:meson-python
 metacomm:AllPairs
 metaphone:Metafone
 metlog:metlog_py