apply methods ("constructors") for Selector in metaprogramming

[erikerlandson]

The Selector type and its sub-classes (SimpleSelector, etc) do not expose any way to construct new selector trees, and this in turn prevents creation of import ... statements in quoted expressions.

Import comes with a constructor:
https://github.com/lampepfl/dotty/blob/d99d9bfc6429f02403c27562fc5f1b5814115e02/library/src/scala/quoted/Quotes.scala#L359

However it requires a List[Selector], and I cannot find a way to construct such a list

[erikerlandson]

cc @armanbilge