Revert "valadoc: Drop duplicated code to add source files and use CodeContext API"
Revert "valadoc: Drop duplicated code to add source files and use CodeContext API"

This reverts commit 54ba304eddd05264cbb9eacd734914938f50c7e4.

Fixes https://gitlab.gnome.org/GNOME/vala/issues/748

frida/vala,GNOME/vala,frida/vala,frida/vala,frida/vala,GNOME/vala,frida/vala,GNOME/vala,GNOME/vala