Move internal quoted.{Expr,Type} implementations to internal package

[nicolasstucki]

These should not be on the public API.

When I add sealed I had to move them to the same file but I did not realize that I could keep them in the correct package.

[liufengyun]

LGTM

Two minor questions (no need to address in this PR):

• do we need to make an upstream PR to sourcecode?
• do we want to add private [scala]?

[nicolasstucki]

I pushed the commit fixing source code directly, therefore no PR is needed.

private [scala] will not work because we use it from within the dotty package.

[smarter]

broke the 2.12 build: http://dotty-ci.epfl.ch/lampepfl/dotty/13353/7

[nicolasstucki]

Will fix it