sysroot: add package_indexes

Add a list of package indexes to SysrootCreateRequest.  If present,
these should be considered for use instead of sourcing the BINHOST.conf
file in the tree.

BUG=chromium:1088059
TEST=generate

Change-Id: I4125e8da6bae492d162f35ec3348a238f2ec172d
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/infra/proto/+/2320736
Commit-Queue: LaMont Jones <lamontjones@chromium.org>
Tested-by: LaMont Jones <lamontjones@chromium.org>
Reviewed-by: George Engelbrecht <engeg@google.com>
Reviewed-by: Alex Klein <saklein@chromium.org>
4 files changed