Add more flexibility to how we import composes.

This change has two main features. The ability to import all trees that are available of a compose
(without failing if some trees are not present), and the ability to select only certain trees from
within a compose.

Bug: 880933
Change-Id: I9ee57f21914efceb69655f8639b72c7522a5f6ae
Merge "Add more flexibility to how we import composes." into develop