pybind11.pc: use pcfiledir for relative destinations (#4830)

* pybind11.pc: use pcfiledir for relative destinations

If the datarootdir is absolute, just use the absolute path directly.
However, if it is relative, we can compute the prefix from the location
of the `.pc` file itself. This allows the install to be relocatable.

* chore: use 3.20's cmake_path if available

* style: pre-commit fixes

* Update CMakeLists.txt

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Co-authored-by: Henry Schreiner <HenrySchreinerIII@gmail.com>
Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>