This is simpler for the user - there is just one uniform lockfile mechanism to know about. And much simpler for us - we can get rid of a ton of code. The drawback for the user is that to create a custom tool lockfile they now also need a requirements.txt (or to add the tool to an existing one) where previously they just made a change in pants.toml. But it was always weird to define requirements in pants.toml instead of in targets, I think.