switch sibling should check for lightweight checkout first

This is based on the following mailing list thread:

http://thread.gmane.org/gmane.comp.version-control.bazaar-ng.general/65367/focus=65433

To quote John Arbash Meinel:

It is because of how the 'trunk' short name is getting looked up. If you
look at bzrlib.builtins.cmd_switch._get_branch_location

It first checks to see if this is a bound branch, before checking if it
is a branch reference. And use the master location as the base for
determining the relative path.

Which is inconsistent with bzrlib.switch._set_branch_location, which
first checks to see if this is a lightweight checkout before checking to
see if it has a master branch.

Probably "just a bug" and we should switch _get_branch_location to check
for a lightweight checkout first.