Migration namespace and path bugs

[vjik]

I added tests with different ways how to specify migration paths and 3 bugs were discovered:

• 1. If a path is specified, the namespace is not substituted;
• 2. If one namespace matches multiple paths, only one path will be found;
• 3. If one path corresponds to several namespaces, the same migrations along this path with different namespaces will be found.

Originally posted by @Tigrov in #333 (comment)