Extracting to temporary directory then renaming to real $wrksrc,
will make the do-extract steps works atomicity. Either $wrksrc is there
and complete, or it's not there.
Accidentally, this change has a side effect, we can no longer care about
the name of top-level components of a tarball, since we will rename the
top level directory in question to $wrksrc. IOW, we don't need to set
$wrksrc any longer. The side effect of above side effect: we can starting
to build multiple packages that have same top-level's name without clean
from now on.
In another hand, we only rename the inner directory if the extracted
file hierarchy has single top-level directory, we will use the
renamed-temporary directory as the $wrksrc, $create_wrksrc variable is
no longer relevant, and do-clean will always work probably instead of
leaving some trash behind like before.
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