[FEATURE] Read migrations from active packages
It is impractical to keep migrations in a distribution-wide folder,
so with this change the migrations are read from all active packages.
Adds the current schema as a MySQL migration as well.
If you have been using FLOW3 in the past, you should have this already.
To mark the migration as already existing run
flow3:doctrine:migrationversion --version 20110613223837 --add