PURL add well-known qualifier vers

[jkowalleck]

As sketched by @pombredanne and discusses in a PURL community meeting,
here is the qualifier vers.

example usage: pkg:cpan/AUTHOR/Module-Name?vers=vers%3Acpan%2F%3E%3D2.2.0%7C%21%3D%202.2.1%7C%3C2.3.0

• closes How to use vers: with pkg:? #386

[jkowalleck]

did not add an example to the spec, as the qualifier encoding is currently reworked -- #398

[mprpic]

Looks good!

[jaimergp]

Is the vers: scheme required or should it be considered redundant? Like, would this be ok?

# with scheme
pkg:cpan/AUTHOR/Module-Name?vers=vers:cpan/>=2.2.0|!= 2.2.1|<2.3.0
# without scheme
pkg:cpan/AUTHOR/Module-Name?vers=cpan/>=2.2.0|!= 2.2.1|<2.3.0

[jkowalleck]

Is the vers: scheme required or should it be considered redundant? Like, would this be ok?

# with scheme
pkg:cpan/AUTHOR/Module-Name?vers=vers:cpan/>=2.2.0|!= 2.2.1|<2.3.0
# without scheme
pkg:cpan/AUTHOR/Module-Name?vers=cpan/>=2.2.0|!= 2.2.1|<2.3.0

thought the same, but went with the full spec for the following reasons:

• the vers: is part of the current version rage spec. keep it in, make adoption easy as no post-processing needed.
• the vers: schema is a good indicator what is going on. maybe version range spec changes in the future, and vers2: would come - so that the schema is a critical indicator/flag.
  Or maybe some contestant version-spec will be released eventually; what would we do then, add another qualifier just for this one, since we don't have clear spec indicators/flags?

this being sayd, the qualifier name vers is debatable. Alternatives i saw:

• version_range --> pkg:cpan/AUTHOR/Module-Name?version_range=vers:cpan/>=2.2.0|!= 2.2.1|<2.3.0
• version_spec --> pkg:cpan/AUTHOR/Module-Name?version_spec=vers:cpan/>=2.2.0|!= 2.2.1|<2.3.0
• tbd

[jkowalleck]

updated/added examples, since #439 was merged