ghc-package.eclass: update registration logis to work with ghc-7.10

The main change is in the '.conf' path. Now we just
register everything in 'src_install()' phase into
local 'package.conf.d' directory and copy it
over into ghc's 'package.conf.d'.

It adds us proper package collision for free,
speeds up ghc reregistration phase.

This mechanism works on ghc-6.12+.

Signed-off-by: Sergei Trofimovich <siarheit@google.com>
dev-haskell/split: allow ghc-7.10

Signed-off-by: Sergei Trofimovich <siarheit@google.com>