dist-git-client should fallback to the traditional [centos] layout for [centos-sig]

[praiskup]

The CentOS SIG documentation allows both layouts, the one with SOURCES and SPECS sub-directories, and the "flat" (like in Fedora).

While the flat layout is probably preferred, and most packages use it, we should be ready to fallback to the traditional CentOS layout with sub-directories in dist-git-client.

Per this matrix thread.

[vanzhiganov]

As I see in example ini-file I can configure Centos layout. But, this is really works incorrect, because dist-git takes SOURCES/ from .{name}.medatada file as filename part, and, as result dist-git truing to get file from incorrect url.