Rework the dependency checks to distinguish required/optional ones

Rework the dependencies definition to include a 'required' flags, which
is passed by the check_dependencies helper to the callers, so that they
can distinguish between required and optional dependencies.

This help in two ways:
- The unit test for the dependencies was now failing when an optional
  one was missing, due to a previous rework.
- Mat2's --check-dependencies was referring to "required dependencies"
  and was misleading for the user as some of them could be optional.

Signed-off-by: Antoine Tenart <antoine.tenart@ack.tf>

tests/test_libmat2.py

@@ -16,7 +16,8 @@ class TestCheckDependencies(unittest.TestCase):
     def test_deps(self):
         ret = check_dependencies()
         for key, value in ret.items():
-            self.assertTrue(value, "The value for %s is False" % key)
+            if value['required']:
+                self.assertTrue(value['found'], "The value for %s is False" % key)
 
 
 class TestParserFactory(unittest.TestCase):