Detect unconditional RDEPEND-RDEPEND cycles

[mgorny]

Unfortunately, since Portage maintainers have historically preferred to add ugly hacks over fixing ebuilds, Portage currently pretends that "pure RDEPEND" cycles aren't there. It would be nice if pkgcheck tries to detect at least unconditional cycles, e.g. A unconditionally RDEP on B, B on C, C on A, and requested developers to use PDEPEND in order to achieve predictable merge order.

[laumann]

CC @laumann

[thesamesam]

Unfortunately, since Portage maintainers have historically preferred to add ugly hacks over fixing ebuilds, Portage currently pretends that "pure RDEPEND" cycles aren't there.

See also https://bugs.gentoo.org/647824.