Reduce duplicated WIT files in the repository

There are a lot of duplicated WIT files in the repository, and it's probably a good idea to either:

• Remove them from source control via ignores (e.g. wit/deps folders, but obviously we need to keep wit/world.wit)
• Centralize and de-duplicate the wit directory contents

This is most egregious in the external test suite (caller + callee components both have separate wit directories), but the reasons it wasn't so straight forward to do:

• symlinks are not well supported by default on windows (so we can't just have the wit dirs be symlinks)
• some toolchains don't deal well with wit dirs that are not local/in the same folder