Treating the first PO file as special was a nice try but some files
don't trigger the creation of a parser, i.e. don't result in the
creation of a rply cache directory and/or cache file.

That's the case if the first file is wiki/src/getting_started.fr.po for
example, which results in various failure modes, depending on who wins
the race:
 - mkdir on the just-created rply directory;
 - trying to read from the just-created and still-empty cache file,
   which is not valid JSON-wise;
 - trying to read from a being-written cache file, leading to
   inconsistent data, meaning another JSON validity problem.

Therefore, consider the XDG_CACHE_HOME environment variable set at the
top-level to be the top-level directory, below which a specific
subdirectory will be created for each i18nspector call. This should
ensure nobody steps on someone else's toes.

This is just an attempt at a minimal bug fix, follow-up commits will
simplify that code drastically.