Changes:
- add timestamp to cache
- add 'valid' flag to package
- make "getMatchingPackages" check for 'valid' flag

1. parse cache; set 'valid' flag of packages to true if called as prt-cache,
   false otherwise
2. if called as prt-cache, return

(non cache)
3. walk prtdir's, for each port
  - stat dir
  - compare timestamps from cache and port directory
    - if they match
?      - compare directories; 
         - if they differ, reparse
         - else set package.valid flag to true (valid cache entry)
    - else
       - set package data back to "" (remember: we use lazy initialization) and
         valid to true


Notes:
might be more efficient to have a separate map<> for the cache, and copy them
over