Set all necessary context before file traversal, not only import
maps. Also, we can skip restoring import maps because all our code is
processed via `For*` iterators which set necessary context.
We can also refactor this a bit to have all context in one place,
this will be done in #2086.
Signed-off-by: Evgeniy Stratonikov <evgeniy@nspcc.ru>
Our current algorithm marks function as used if it is called
at least ones, even if the callee function is itself unused.
This commit implements more clever traversal to collect usage
information more precisely.
Signed-off-by: Evgeniy Stratonikov <evgeniy@nspcc.ru>