gen-qstr.sh

Previously, we left out the dependency of the QSTR header on
`modules.h`.  This was done to prevent rebuilds of the entire
MicroPython sources whenever a Pycardium module is changed. This
leads to issues where QSTRs got out of sync and weird errors like
the following could happen:

    import foo_module

    Exception: No module `abc_def` (a different string than expected)

Attempt to fix this by only updating the QSTR header when the
module-header actually changes.  For this, a few workarounds are needed:

- Replace symlinks with actual copied files so timestamps change on
  updates.
- Add a hack so meson picks up on the dependency of the file in genhdr/
- Rename the outer file so older meson versions don't complain about
  multiple targets with the same name.

Co-authored-by: dx <dequis@dequis.org>
Signed-off-by: Rahix <rahix@rahix.de>