base.bbclass: not use bb.fetch.urldata in do_unpack() for traversing SRC_URIs

base.bbclass: not use bb.fetch.urldata in do_unpack() for traversing SRC_URIs

bb.fetch.urldata contains cached SRC_URIs, but in case SRC_URI is altered by
amend.inc or machine override, the original SRC_URI is not removed from the
cache, leading to calling do_unpack() multiple times on original and modified
versions of the file. Don't rely on bb.fetch.urldata and traverse SRC_URIs
directly.

Some background and details:
http://thread.gmane.org/gmane.comp.handhelds.openembedded/36703

Signed-off-by: Denys Dmytriyenko <denis@denix.org>
Acked-by: Chris Larson <chris_larson@mentor.com>
Acked-by: Khem Raj <raj.khem@gmail.com>
Acked-by: Frans Meulenbroeks <fransmeulenbroeks@gmail.com>