Fix: doc: enable doxygen comment blocks in source under lib

Previously doxygen public API documentation would be produced only if the
comment blocks were in the header files. Now it will use comment blocks in
source files as well. This also enables viewing indexed source code within the
doxygen documentation and showing more include graphs.

Non-public APIs are unaffected because INPUT only controls what is processed,
not what is documented. However, both public and private source files and
headers under the include and lib directories are now viewable in the indexed
source code, and private data structures do show up under "Data Structures".