Update the lock file when necessary

[rtimush]

The PR is created as a draft, as it includes changes from #109.

Closes #83.

CLI changes

• protofetch fetch: updates protofetch.lock if protofetch.toml has changed. Previous behavior: ignore protofetch.toml even if changed, fetch whatever protofetch.lock says.
• protofetch fetch --locked: verifies that the lock file is up-to-date and fails otherwise
• protofetch fetch --force-lock: deprecated, but still works for backwards compatibility.
• protofetch update: recreates the lock file, like fetch --force-lock used to, but does not actually fetch the protos

It is possible to keep --force-lock, but I think it'd better to deprecate this flag:

• it's poorly named, the name looks like it forces protofetch to use the lock file, but in fact it's the opposite
• it combines two steps (recreating the lock file, and fetching the protos), which should be rarely needed. It should be more common to only want to recreate the lock file, and fetching the protos is usually a part of some build script.

API changes

fetch and lock now accept a LockMode enum.

Compatibility

• older versions of protofetch seamlessly use the new lock files
• the new versions of protofetch will have to update old lock files, fetch --locked will not succeed unless the updated lock file is available.