Fix sorting of plugins with dependencies

[domekomi]

Fix a problem that a plugin with deep dependencies is not sorted properly. This PR also add features to detect errors of circular dependencies and missing plugins.

[tyru]

Sorry for late reply.
Could you show me the examples?

• deep dependencies is not sorted properly
• circular dependencies
• missing plugins

[domekomi]

deep dependencies is not sorted properly

If a plugin A depends on B and B doesn't depends on other plugins (A -> B), these plugins are sorted properly. However, if B depends on C (A -> B -> C), not sorted properly.

circular dependencies

e.g. A depends on B and B depends on A (A -> B -> A).

missing plugins

e.g. A depends on B, but B is not found.

[tyru]

Should be related #333