[wip] RDEPEND cycle check

[arthurzam]

This is WIP, showing results across the repo, but with possible false positives (since we compare on cat/pkg level instead of cat/pkg-ver levels).

Resolves: #400

[thesamesam]

• net-libs/gssdp has a FP: gssdp-1.6.2 depends on gssdp-1.4:0, but it thinks there's a cycle
• dev-libs/libusb-compat has a FP: libusb-compat-0.1.5-r3 depends on virtual/libusb:1, which depends on >=dev-libs/libusb-1.0.19:1 (so it's ignoring slots, or it's confused by the blocker in libusb-compat; we should ignore blockers for now, that's subject of another bug: Checking for blockers without a solution #507)

not checked others yet

[thesamesam]

It looks good so far, but not tried it yet.