Inline generated export headers

[benjaminwinger]

#2118 broke the single file header generator script because it adds a cmake-generated header which resides in the build directory (probably should add tests to CI for the header generator at some point). This adds the build directory to the list of directories to search for header files when creating the single file header.

[codecov]

Codecov Report

All modified lines are covered by tests ✅

Comparison is base (86d0c96) 89.61% compared to head (e87c39b) 89.61%.
Report is 7 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #2124   +/-   ##
=======================================
  Coverage   89.61%   89.61%           
=======================================
  Files         988      988           
  Lines       35644    35645    +1     
=======================================
+ Hits        31944    31945    +1     
  Misses       3700     3700           

see 5 files with indirect coverage changes

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[benjaminwinger]

Re-written to just inline the headers since this also breaks the single file C header which gets included verbatim without substituting includes.