perl5 portgroup: resolve version ambiguity with subports

[jmr@…]

Currently, perl modules are built against whatever version of perl happens to be linked to ${prefix}/bin/perl. This causes much confusion and is incompatible with prebuilt binaries.

Attached is a proposed change. It takes a similar approach to the new unified python portgroup, creating several p5.X-foo ports from each p5-foo portfile. This way, p5.X-foo is known to be built against perl5.X. The p5-foo port itself becomes a metaport that pulls in one of the specific versions.

[jmr@…]

Might as well do #11873 at the same time since every p5 port will need to be touched anyway.

[jmr@…]

r82547