SI-7070 Turn restriction on companions in pkg objs into warning

The implementation restriction created from SI-5954 in
3ef487ecb6733bfe3c13d89780ebcfc81f9a5ea0 has two problems.

1) The problematic code works fine if compile with sbt. That means the
restriction is breaking some people needlessly.

2) It's not binary compatible.

To fix all that this commit changes the error into a warning and removes
the setting used to get around the restriction.