Add script to gather git submodules

[Flamefire]

Running this on a recursively cloned git repository will show all submodules, its path and commit hash formatted as a Python dict which can be used in easyconfigs.

I needed this recently for an easyconfig.
Result was:

 {
    'amqpcpp': ('CopernicaMarketingSoftware/AMQP-CPP', '6ddabe4', 'lib/metricq/lib/amqpcpp'),
    'asio': ('chriskohlhoff/asio', 'fa27820c0', 'lib/metricq/lib/asio'),
    'Catch': ('catchorg/Catch2', '7c9f92bc', 'lib/metricq/lib/nitro/tests/Catch'),
    'Catch': ('catchorg/Catch2', '7c9f92bc', 'lib/plugin_cxx_wrapper/lib/nitro/tests/Catch'),
    'common': ('mem/plugin_common', '1aa933b', 'lib/plugin_cxx_wrapper/common'),
    'date': ('HowardHinnant/date', '6e921e1', 'lib/metricq/lib/date'),
    'fmt': ('fmtlib/fmt', '7bdf0628', 'lib/metricq/lib/fmt'),
    'metricq': ('metricq/metricq-cpp', '40018cb', 'lib/metricq'),
    'metricq-protobuf': ('metricq/metricq-protobuf', '733b836', 'lib/metricq/lib/metricq-protobuf'),
    'nitro': ('energy/nitro', 'ac613d6', 'lib/plugin_cxx_wrapper/lib/nitro'),
    'nitro': ('energy/nitro', 'e117e60', 'lib/metricq/lib/nitro'),
    'plugin_cxx_wrapper': ('mem/plugin_cxx_wrapper', '7adb5a9', 'lib/plugin_cxx_wrapper'),
}

[boegel]

@Flamefire Please include a license header, which also mentions author

[Flamefire]

Done