CMake: Add SDFTD3_INSTALL_FMODDIR

[Growl1234]

This is mainly for convenience of Fedora packaging as it requires Fortran modules to be installed under %{fmoddir}.

Note: We shouldn't simply adopt what's done for mctc-lib etc:

set(
  "${PROJECT_NAME}-module-dir"
  "${PROJECT_NAME}/modules"
  CACHE STRING
  "Subdirectory to install generated module files to"
)
set(
  module-dir
  "${${PROJECT_NAME}-module-dir}"
)
set(module-dir "${module-dir}" PARENT_SCOPE)

Downstream sets CMAKE_INSTALL_INCLUDEDIR to %{_lib}/gfortran/modules as a workaround, which is not applicable to s-dftd3 as it has public C header. This is also an issue on dftd4 and tblite side; will fix them soon.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 65.28%. Comparing base (5bf48d7) to head (89ded45).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #239      +/-   ##
==========================================
+ Coverage   63.49%   65.28%   +1.78%     
==========================================
  Files          47       40       -7     
  Lines        6821     5919     -902     
  Branches     2422     2422              
==========================================
- Hits         4331     3864     -467     
+ Misses       1182      747     -435     
  Partials     1308     1308              

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