added spec for opkg PURL type

[oblivia-simplex]

I wrote up a spec for the opkg PURL type, which I think we should use as a root type instead of openwrt (which is currently stubbed out but not further specified in the packageurl-go repo, for example). I suggest letting openwrt act as a namespace for opkg PURLs, which is consistent with what the spec already has for apk PURLs, as explained in PURL-TYPES.rst.

apk

apk for APK-based packages:

• There is no default package repository: this should be implied either from
  the distro qualifiers key or using a repository base url as
  repository_url qualifiers key.

• The namespace is the vendor such as alpine or openwrt. It is not
  case sensitive and must be lowercased.

• The name is the package name. It is not case sensitive and must be
  lowercased.

• The version is a package version as expected by apk.

• The arch is the qualifiers key for a package architecture.

• Examples::

  pkg:apk/alpine/[email protected]?arch=x86
  pkg:apk/alpine/[email protected]?arch=x86
  

One potentially contentious detail in my opkg PURL spec is the qualifier key I called set, which corresponds to what opkg calls "repositories". I chose set as a name for this field rather that repository or repo to avoid confusion with the repository_url field. The "sets" in question, in the case of OpenWrt's opkg distribution, include

• base
• community-packages
• luci
• routing
• telephony

The key set was chosen by analogy with OpenBSD's "sets" (collections of related packages, with names like "base", "game", "font", etc.). I'm not particularly wedding to this choice, though, and would welcome alternative suggestions.

[oblivia-simplex]

Unrelatedly, I moved the qpkg section to its proper alphabetical place in the file.