Implement importutils.try_import.

Adds a new (simple) importutils.try_import function which
can return a module name or a default (by default this is None).

This should help clean up some of our

try:
    import foo
except...

blocks in modules.

This commit also drops the dependency on python-extras which is
no longer needed.

Fixes LP Bug #1099501.

Change-Id: I8591f79983bdad67a50d1aaae6dce8428dfce084
Merge "Implement importutils.try_import."