Added support for specifying aliases for migrationNamespaces
Added support for specifying aliases for migrationNamespaces

This is used to specify pathes to migrations that do not have
namespaces.

While not directly supported by the migration command provideded by the
framework, these migrations exist in a lot of extensions because custom implementations
of migration controllers out of the framework were using this approach
to load multiple migrations from multiple paths.

Even the framework itself currently ships non-namespaced migrations:

- https://github.com/yiisoft/yii2/blob/17a1d91e4a517f4f15dce973bf3c50dd939dce63/framework/rbac/migrations/m140506_102106_rbac_init.php
- https://github.com/yiisoft/yii2/blob/17a1d91e4a517f4f15dce973bf3c50dd939dce63/framework/caching/migrations/m150909_153426_cache_init.php
- https://github.com/yiisoft/yii2/blob/17a1d91e4a517f4f15dce973bf3c50dd939dce63/framework/log/migrations/m141106_185632_log_init.php

This change allows existing applications to adopt the new namespace-based approach
while keeping existing migrations. While it would be possible to add
namespaces to migrations in the application itself, it is not easily possible
to add namespaces to migrations that come from external sources like
extensions.

yiisoft/yii2-framework,yiisoft/yii2-framework