Test demonstrating SI-8085

* The presentation compiler sourcepath is now correctly set-up.
* Amazingly enough (for me at least), the outer import in the
  package object seem to be responsible of the faulty behavior.
  In fact, if you move the import clause *inside* the package object,
  the test succeed!
  The test's output does provide the correct hint of this:

```
% diff /Users/mirco/Projects/ide/scala/test/files/presentation/t8085-presentation.log /Users/mirco/Projects/ide/scala/test/files/presentation/t8085.check
@@ -1,3 +1,2 @@
 reload: NodeScalaSuite.scala
-prefixes differ: <empty>.nodescala,nodescala
-value always is not a member of object scala.concurrent.Future
+Test OK
```

Notice the ``-prefixes differ: <empty>.nodescala,nodescala``. And compare
it with the output when the import clause is placed inside the package
object:

```
% diff /Users/mirco/Projects/ide/scala/test/files/presentation/t8085-presentation.log /Users/mirco/Projects/ide/scala/test/files/presentation/t8085.check
@@ -1,4 +1,2 @@
 reload: NodeScalaSuite.scala
-reload: NodeScalaSuite.scala
-open package module: package object nodescala
 Test OK
```

Notice now the ``-open package module: package object nodescala``!
SI-8085 Fix BrowserTraverser for package objects

A source file like:

    import foo.bar
    package object baz

Is parsed into:

    package <empty> {
       import foo.bar
       package baz {
          object `package`
       }
    }

A special case in Namers compensates by adjusting the owner of
`baz` to be `<root>`, rather than `<empty>`.

This wasn't being accounted for in `BrowserTraverser`, which
underpins `-sourcepath`, and allows the presentation compiler to
load top level symbols from sources outside those passes as
the list of sources to compile.

This bug did not appear in sources like:

    package p1
    package object p2 { ... }

... because the parser does not wrap this in the `package <empty> {}`

This goes some way to explaining why it has gone unnoticed for
so long.
Merge pull request #3282 from retronym/ticket/8085

 Fix BrowserTraverser for package objects