break out categories.txt generation to standalone method

break out categories.txt generation to standalone method

This is to clean up the code for the introduction of a new index format.
This also makes it so that only repo/categories.txt is generated, and not
archive/categories.txt.

Currently, the only thing that I can find that reads categories.txt is the
wordpress plugin, e.g. wp-fdroid.  And it only reads repo/categories.txt
not archive/categories.txt.