Fix #10182 by disallowing/avoiding self {-# SOURCE #-} imports

Summary:
hs-boot declarations were leaking into the EPS due to
self {-# SOURCE #-} imports, and interface loading induced by
orphan instances.  For the former, we simply disallow self
{-# SOURCE #-} imports; for the latter, we simply just don't
load an interface if it would be ourself.

Signed-off-by: Edward Z. Yang <ezyang@cs.stanford.edu>

Test Plan: validate

Reviewers: simonpj, austin

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D860

GHC Trac Issues: #10182