oeqa/runtime/context: remove duplicate sys.path entries when looking for modules

oeqa/runtime/context: remove duplicate sys.path entries when looking for modules

sys.path can contain duplicate entries for each layer, which means that
the search in add_controller_list() will find the same name twice and
abort.

As duplicate directories should be harmless, remove any duplicates before
iterating through the entries.

Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>