plugin: add stub of code for handling conflicts

Signed-off-by: Rafał Malinowski <rafal.przemyslaw.malinowski@gmail.com>
plugin: use tuples instead of pairs in tst_PluginDependencyGraphBuilder

Signed-off-by: Rafał Malinowski <rafal.przemyslaw.malinowski@gmail.com>
plugin: add "provides" to testing tuple of plugins

Signed-off-by: Rafał Malinowski <rafal.przemyslaw.malinowski@gmail.com>
plugin: add more tests (not passing) for PluginDependencyGraphBuilder

Signed-off-by: Rafał Malinowski <rafal.przemyslaw.malinowski@gmail.com>
plugin: rework PluginDependencyGraphBuilder class

Signed-off-by: Rafał Malinowski <rafal.przemyslaw.malinowski@gmail.com>