Reloading of flow spanning multiple files

[simonafk]

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If a flow span multiple files (e.g. a flow merges another flow from a different module), reloading may not work correctly.

Possible issues

• Reload ordering: If a flow X merges flow Y from another module, then the module containing flow Y needs to be reloaded before the module containing flow X.
• Dependency changes: Flow dependencies may be changed (e.g. flow X previously merges flow Y, but now merges flow Z instead), it is possible that the modules for the newly added dependencies need to be reloaded.

We may need to implement recursive module reloading.

[simonafk]

IPython has a recursive module reloading implementation: https://github.com/ipython/ipython/blob/master/IPython/lib/deepreload.py