SI-8862 Fix treatment of inherited implicits in package objects

Two spots in implicit search fell prey to a trap with package
objects. Members of a package object are entered into the scope
of the enclosing package, but that doesn't form a suitable prefix
for determing the member type.

A REPL transcript paints a picture that speaks a 1000 words:

```
scala> :paste -raw
// Entering paste mode (ctrl-D to finish)

package p { class C[A] { def foo: A = ??? }; object `package` extends C[String] }

// Exiting paste mode, now interpreting.

scala> val p = rootMirror.getPackageIfDefined("p")
warning: there was one deprecation warning; re-run with -deprecation for details
p: $r.intp.global.Symbol = package p

scala> p.info.decls
res0: $r.intp.global.Scope = Scopes(class C, package object p, method foo)

scala> val foo = p.info.decl(TermName("foo"))
foo: $r.intp.global.Symbol = method foo

scala> p.typeOfThis memberType foo
res1: $r.intp.global.Type = => A

scala> val fooOwner = foo.owner
fooOwner: $r.intp.global.Symbol = class C

scala> p.info.decl(nme.PACKAGE).typeOfThis memberType foo
res3: $r.intp.global.Type = => String
```

This commit detects if we find an implicit in a package module,
and then uses the self type of the corresponding package object
as the prefix for the `ImplicitInfo`. This is done in both
`Context.implicitss` (which collects in-scope implicits), and
in `companionImplicitMap` (which harvests them from the implicit
scope.) In addition, it was necessary / possible to remove a special
case that excluded package object implicits, the referenced tests for
SI-3999 now pass without this.
You have just won spot in "Great Moments in Code Review, Package Object edition".

```scala
package p {
  trait T[X] { def O : { def apply(): X } }
  object `package` extends T[Int] {
    def O: { def apply(): Int } = new { def apply(): Int = 42 }
  }

  object Test {
    def main(args: Array[String]): Unit = {
      val x: Int = O()
    }
  }
}
```

```
% qscalac sandbox/test.scala
sandbox/test.scala:10: error: type mismatch;
 found   : X
 required: Int
      val x: Int = O()
                    ^
one error found
```