compiler: change expression importing to use Import_expression

compiler: change expression importing to use Import_expression

    Change expression importing to use a new abstract interface class
    Import_expression, so that we can more easily import expressions from
    inlinable function bodies.  This is a refactoring with no affect on
    compiler behavior.

    Reviewed-on: https://go-review.googlesource.com/c/150065

From-SVN: r266526