Make comparisons in is_cached independent of order

[skshetry]

On is_cached, we used to compare two dicts: one of current stage in memory
created by run and one that's already written to the file.
As outs and dicts are lists (which is generate by dumpd), the
comparison was dependent on the order of outs and deps.

So, the run before and now, can be in different order and would fail the comparisons.

Also, given that we can compare without order, we can make pipeline files sorted by outs and deps.

Plus, in the post-pipeline-file world, we don't even need is_cached. We could just throw
DuplicateStage or OutputDuplicationError. It's convenient of course, to be able to dvc run
same command many times but, I'm not sure if it's worth the hassle of having is_cached for just that.

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[efiop]

Do we need a test for this?

[skshetry]

@efiop, we already have a test for pipeline stage that should cover this (tests.func.test_run_multistage.test_multi_stage_run_cached).

The external behavior that needs to be tested is following:

dvc run  -d foo0 -d foo1 -o bar0 -o bar1 "cp foo0 bar0 && cp foo1 bar1"
dvc run  -d foo1 -d foo0 -o bar0 -o bar1 "cp foo0 bar0 && cp foo1 bar1"

Previous behavior considered these to be different, and would ask for overwriting the file bar0.dvc it would output. Now, it'll be considered as cached.