libkmod-index: keep index and comments in .c

Just like other source files, keep the index and comments in the source
file rather than the header.

This also removes INDEX_PRIORITY_MIN that was never being used.
depmod: point to documentation in libkmod

Instead of repeating all documentation, point to the documentation
available in libkmod-index.c

This also removes INDEX_PRIORITY_MIN that was not being used.
libkmod-index: remove invalid comment
libkmod-index: move comment to include the includes