improve VCS packages checks

* Unlike older version, currently service will always try to pull AUR
  package to check version. Previously if no-vcs flag is set, it would
  ignore VCS packages completelly
* Introduce build.vcs_allowed_age option. If set, it will skip version
  calculation if package age (now - build_date) is less than this value

package/share/ahriman/settings/ahriman.ini

@@ -24,6 +24,7 @@ ignore_packages =
 makechrootpkg_flags =
 makepkg_flags = --nocolor --ignorearch
 triggers = ahriman.core.report.ReportTrigger ahriman.core.upload.UploadTrigger
+vcs_allowed_age = 604800
 
 [repository]
 name = aur-clone