Add a script that downloads dependencies.

[thibautbuchert]

This PR adds a script that downloads dependencies. It ended up looking a bit like git submodules, excepts that dependencies are downloaded outside the working copy.

All the dependencies for a given project are stored as a small configuration file with one dependency per line. Each line contains the working copy path, the repository URL, and the commit that will be checked out. working copy paths are relative to the current directory, so for example in the case of this pilot episode you would run the script from the directory above the working copy of Pilot_Episode like this:

Pilot_Episode/fetch-dependencies.py Pilot_Episode/dependencies

I did not use the working copy of Pilot_Episode as the base directory for dependency working copy paths because I do not like relative paths that contain ...

A few points about the script:

• It requires a recent Python version, at least 3.10 (that supports pattern matching)
• I don't think that specifying branches instead of commit hashes in the configuration file will work (an if it works, it is not intentional). Branches can change over time and it is better to always have the same commit checked out when fetching the dependencies for an older commit of the Pilot_Episode. In the future I may try to add an option that will make the script try to checkout a branch that points to the given commit if there is one.
• The script will not do anything for a dependency if it looks like local changes would be destroyed.

[thibautbuchert]

@razterizer Seems that I don't have any right to add you as a reviewer. What do you think of this script ?

[thibautbuchert]

I will also update the README

[thibautbuchert]

I don't understand half of what you've done so I guess there's not much to comment on. I think perhaps I should just pull it and test it out on this new branch and see how it works in action.

If you could test it on mac that could be great !