PurlDB: purl2sym Data collection Batch6

[pombredanne]

Here we add purl2sym support for a list of PURLs listed below.

1. Add support in FetchCode and PurlDB
2. Test of end-to-end support in the PurlDB

• pkg:generic/opkg
  • There are too many versions floating around: Yocto has a one, forked from OpenWRT, OpenWRT has one and all are forked from Lede itself a fork of openwrt, that later rejoined. This ends up being complex.