Fix forward declarations with directory packages

[bkryza]

Fixes #390

[codecov]

Codecov Report

Attention: Patch coverage is 97.63780% with 3 lines in your changes missing coverage. Please review.

Project coverage is 90.24%. Comparing base (b473fc3) to head (cb3ea37).
Report is 6 commits behind head on master.

Files with missing lines	Patch %	Lines
src/common/model/element.cc	0.00%	2 Missing ⚠️
src/class_diagram/model/diagram.h	92.30%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #391      +/-   ##
==========================================
- Coverage   90.26%   90.24%   -0.03%     
==========================================
  Files         152      152              
  Lines       19074    19033      -41     
==========================================
- Hits        17217    17176      -41     
  Misses       1857     1857              

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