eolian: refactor parsing API and path handling
This splits the eolian_file_parse API into two, one for parsing
files already present in the database (always by filename) and
one for parsing paths.
It fixes several bugs/leaks on the way (incorrect use of
stringshare etc.) as well as adds checking for whether there
are no conflicting filenames at scan time, for free. That means
it is now no longer possible to scan two paths which have an eo
or eot file of the same name in them.
It should also be faster now.
It also fixes T7820.