Unify the definitions of the two function to take a set of files instead
of different arguments. The list of files has the form:
files = { "full/path/name.md" = "name.md" }
I.e. the complete path is the key and the file basename the value,
leading to a set of files, even if basenames are the same.
Usually it will be created by the get_all_files function of the same
module, but it can be passed in manually as well (e.g. to avoid
duplicate filesystem lookups, etc).
Renamed to 'files.lua' since it is the only module actively working on
the underlying filesystem so far -- gathering existing files and zettel
in the directories.