meson: use install_subdir

[zoziha]

Description

• When minpack is the main meson project, using install-mod.py to install the *.mod file, meson install will be expected.
• When minpack is a meson subprojects project, installing the *.mod file using install-mod.py will have an unexpected effect.

The solution is to use meson's install_subdir, which I have used in several of my own projects and it meets all the requirements for a normal *.mod install.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.80%. Comparing base (c0b5aea) to head (0627e39).

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #79   +/-   ##
=======================================
  Coverage   88.80%   88.80%           
=======================================
  Files           2        2           
  Lines        1224     1224           
  Branches      456      456           
=======================================
  Hits         1087     1087           
  Misses         40       40           
  Partials       97       97           

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