This commit moves code to look for vars files/dirs to a common place and
uses it for loading role defaults/vars. This allows things such as
'defaults/main' or 'vars/main' being a directory in a role, allowing
splitting of defaults/vars into multiple files. This commit also fixes
the role loading unit tests for py3 when bytestrings are used for paths
instead of utf8 strings.
This fixes#14248 and #11639.
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