reduce CLI startup overhead by importing less

[davidsarah]

bin/tahoe takes about 3.5 seconds to perform a trivial command (e.g. bin/tahoe --help) on my machine. Most of this overhead is due to importing dependencies: src/allmydata/__init__.py​ imports all of Tahoe's dependencies, in order to check their versions.

Just removing the calls to get_package_versions_and_locations() and check_all_requirements() does not work; we end up importing most of these libraries anyway. However, most of the CLI commands that do not start a node, should not need to import anything except twisted.python.usage.

By copying just the files that are needed for the CLI to work at all, and removing imports that are not really necessary, I was able to get the time to run bin/tahoe --help on my machine to 1.1 seconds, i.e. a factor-of-3 improvement. This is just a proof of concept at the moment, but I think this improvement could also be obtained for the real CLI. Note that just running a script that prints out the help text takes 0.8 seconds, due to the startup overhead of the Python interpreter.

Another major advantage of doing this is that the resulting CLI could be embedded as a library, without pulling in any of Tahoe's dependencies and their associated packaging woes.