rpm: New RPM database location in /usr/lib/sysimage/rpm

A few years ago the RPM database was moved from /var to /usr.  This is
now feeding through to Linux distros.

http://lists.rpm.org/pipermail/rpm-maint/2017-October/006723.html
https://fedoraproject.org/wiki/Changes/RelocateRPMToUsr
https://src.fedoraproject.org/rpms/rpm/pull-request/21

src/ph_rpm.ml

@@ -236,12 +236,22 @@ let rpm_package_name pkg =
   let rpm = rpm_of_pkg pkg in
   rpm.name
 
+let rpmdb_locations = [
+  "/usr/lib/sysimage/rpm/rpmdb.sqlite";
+  "/var/lib/rpm/rpmdb.sqlite";
+  "/var/lib/rpm/Packages"
+]
+
+let find_rpmdb () =
+  let rec loop = function
+    | [] -> error "rpm: cannot locate RPM database; if this is a normal RPM-based Linux distro then this is probably a supermin bug"
+    | db :: rest ->
+       if Sys.file_exists db then db else loop rest
+  in
+  loop rpmdb_locations
+
 let rpm_get_package_database_mtime () =
-  (try
-    lstat "/var/lib/rpm/rpmdb.sqlite"
-   with Unix_error (ENOENT, _, _) ->
-    lstat "/var/lib/rpm/Packages"
-   ).st_mtime
+  (lstat (find_rpmdb ())).st_mtime
 
 (* Return the best provider of a particular RPM requirement.
  *