bitbake: layerindexlib: Fix parsing of recursive layer dependencies

bitbake: layerindexlib: Fix parsing of recursive layer dependencies

[YOCTO #13447]

When running bitbake-layers layerindex-fetch from 'master', there is a
circular dependency between meta-oe and meta-python.  This triggered a maximum
recursion depth exception.

To fix the exception, as we walk down a branch (depth first search), we track
the layers we've already seen.  If we are about to recurse into a layer we've
already seen we report a warning and then stop recursion.

(Bitbake rev: d6155d095513be3f500d089c4ed4c4b89949d560)

Signed-off-by: Mark Hatle <mark.hatle@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>