support/graph-depends: detect circular dependencies

Currently, if there is a circular dependency in the packages, the
graph-depends script just errors out with a Python RuntimeError which is
not caught, resulting in a very-long backtrace which does not provide
any hint as what the real issue is (even if "RuntimeError: maximum
recursion depth exceeded" is a pretty good hint at it).

We fix that by recursing the dependency chain of each package, until we
either end up with a package with no dependency, or with a package
already seen along the current dependency chain.

We need to introduce a new function, check_circular_deps(), because we
can't re-use the existing ones:

  - remove_mandatory_deps() does not iterate,

  - remove_transitive_deps() does iterate, but we do not call it for the
    top-level package if it is not 'all'

  - it does not make sense to use those functions anyway, as they were
    not designed to _check_ but to _act_ on the dependency chain.

Since we've had time-related issues in the past, we do not want to
introduce yet another time-hog, so here are timings with the circular
dependency check:

    $ time python -m cProfile -s cumtime support/scripts/graph-depends
    [...]
             28352654 function calls (20323050 primitive calls) in 87.292 seconds

       Ordered by: cumulative time

       ncalls  tottime  percall  cumtime  percall filename:lineno(function)
            1    0.012    0.012   87.292   87.292 graph-depends:24(<module>)
           21    0.000    0.000   73.685    3.509 subprocess.py:473(_eintr_retry_call)
            7    0.000    0.000   73.655   10.522 subprocess.py:768(communicate)
            7   73.653   10.522   73.653   10.522 {method 'read' of 'file' objects}
          5/1    0.027    0.005   43.488   43.488 graph-depends:164(get_all_depends)
            5    0.003    0.001   43.458    8.692 graph-depends:135(get_depends)
            1    0.001    0.001   25.712   25.712 graph-depends:98(get_version)
            1    0.001    0.001   13.457   13.457 graph-depends:337(remove_extra_deps)
         1717    1.672    0.001   13.050    0.008 graph-depends:290(remove_transitive_deps)
    9784086/2672326    5.079    0.000   11.363    0.000 graph-depends:274(is_dep)
    2883343/1980154    2.650    0.000    6.942    0.000 graph-depends:262(is_dep_uncached)
            1    0.000    0.000    4.529    4.529 graph-depends:121(get_targets)
      2883343    1.123    0.000    1.851    0.000 graph-depends:246(is_dep_cache_insert)
      9784086    1.783    0.000    1.783    0.000 graph-depends:255(is_dep_cache_lookup)
      2881580    0.728    0.000    0.728    0.000 {method 'update' of 'dict' objects}
            1    0.001    0.001    0.405    0.405 graph-depends:311(check_circular_deps)
    12264/1717    0.290    0.000    0.404    0.000 graph-depends:312(recurse)
    [...]
    real    1m27.371s
    user    1m15.075s
    sys     0m12.673s

The cumulative time spent in check_circular_deps is just below 0.5s,
which is largely less than 1% of the total run time.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Cc: Samuel Martin <s.martin49@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

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