SI-8207 Allow import qualified by self reference

This regressed in SI-6815 / #2374. We check if the result of
`typedQualifier(Ident(selfReference))` is a stable identifier
pattern. But we actually see the expansion to `C.this`, which
doesn't qualify.

This commit adds a special cases to `importSig` to compensate.
This is safe enough, because the syntax prevents the following:

	scala> class C { import C.this.toString }
	<console>:1: error: '.' expected but '}' found.
	       class C { import C.this.toString }
	                                        ^
So loosening the check here doesn't admit invalid programs.
I've backed this up with a `neg` test.

The enclosed test also checks that we can use the self
reference in a singleton type. (That wasn't broken.)

Maybe it would be more principled to avoid expanding the self
reference in `typedIdent`. I can imagine that the current situation
is a pain for refactoring tools that try to implement a rename
refactoring, for example.

Seems a bit risky at the minute, but I've noted the idea
in a comment.