Avoid duplicated dependencies in vs201x toolsets output

[vadz]

Although nobody would arguably duplicate the dependencies on purpose, it
was easy to do this unintentionally by inheriting a dependency from a
base template and also adding it explicitly -- and this, in fact,
happened in several places in real projects, resulting in not quite
correct solution and project files being generated: such files could
still be read by MSVS, but were rewritten by it to prune the duplicate
dependencies, resulting in changes compared to the originally generated
versions.

Avoid this by emitting the dependencies only once in both the solution
and the project files: this requires duplicating the duplication pruning
logic, but is better than trying to do it at the model level because the
dependencies may result from the evaluation of expressions involving the
toolset name and other variables, so this approach is much simpler to
make work.

Also add a unit test verifying that this now behaves as expected.

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This replaces #104.

[vslavik]

No other comments, I think this is the better fix overall.

[vslavik]

My first draft said then it would probably be simpler to instead write

for d in OrderedSet(prj.dependencies):
    ...

But then I grepped the code for such uses and found something better:

from bkl.utils import filter_duplicates
...
for d in filter_duplicates(prj.dependencies):
    …do something… 

Probably doesn't even need the explanatory comments then — it says what it does and it's clear why one might want to eliminate duplicates here...

[vadz]

Great, thanks for the filter_duplicates() hint!

I'd still keep the comment because it might not be obvious that it's more than just an "optimization".