WIP: Add support for importing build dependencies in sysroots.

Description

This feature was proposed in August in the mailing list: https://mail.gnome.org/archives/buildstream-list/2018-August/msg00009.html

This feature allow to stage build dependencies in a different root directory. This does not affect runtime dependencies.

However runtime dependencies of build dependencies of build dependencies with sysroot will end up in the same sysroot.

Integration commands are not run for build dependencies with sysroots.

To avoid duplication of text for the sysroot path, the format has been changed from the proposal to group dependencies by sysroot.

This feature is used in Freedesktop SDK branch valentindavid/sysroot_dependencies