New feature: allow origins of all dependencies be recorded into package list

using new `@comment DEPORIGIN:...' directive. This would allow us to make
many neat things including:

- easier binary upgrades;
- source upgrades without using external tools by simply extending
  bsd.port.mk and pkg_install tools;
- mixed-mode upgrades (source + binary);
- depreciate and deorbit silly +REQUIRED_BY files in the near future.

This feature is no-op until appropriate bsd.port.mk patch is committed, and
even when it is already committed packages generated will remain 100%
compatible with old set of pkg_install tools (module all those neat
features, of course).

MFC after:	6 days

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