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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.
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