* Move share/qt5/doc/* in qtbase from -main to a separate subpackage,
-global, which (as a bonus) "owns" share/doc/qt5/ directory;
* Move FOO.index files from -html to a separate subpackage, -docindex,
and make 'em depend on qtbase,-global;
* Make DEP,-docindex automatically added as BDEP for each DEP in
LDEP and BDEP mentioned for -main;
* Make it clear that building -qch without -html is impossible;
While there, fixed a few minor issues.
This make at least qtenginio,-html build correctly, which failed before
due to missing .index files during qdoc run. Initial report from kili@.
