[gentoo-dev] vcs-snapshot.eclass: add a way to specify the extraction path

[William Hubbs]

All,

I'm finding in working on Go ebuilds, that we are propegating a
src_unpack function that is very similar to the one in vcs-snapshot.

This patch adds an EXTRACT_PATH variable to the vcs-snapshot eclass
which, if set, puts the extracted archives in the specified directory under ${S}.

If it is not set, nothing should happen.

This could be used by other types of ebuilds later, but for now it would
be used by Go ebuilds.

Thoughts?

William