[tool] Allow running from anywhere (#4199)

Now that the repo tooling is always run from source, not via `pub global`, we no longer need to infer the repo location from the current directory. Instead, hard-code knowledge of where the repository root is. This makes it much easier to run the tooling, since it's common to be in a package directory rather than the repo root.

To make it even easier to run from within a package, this also adds a `--current-package` as an alternative to `--packages`. This makes it possible to, e.g., write local wrapper scripts that run a specific set of commands on whatever the current package happens to be (such as a generic version of the script discussed in #4129).

As related cleanup, makes the tool non-publishable (we haven't been publishing it since the repo merge, but I never made it unpublishable; this is important now that it would not work if published) and remove the LICENSE and CHANGELOG since it's no longer a stand-alone package.

Fixes flutter/flutter#128231
Fixes flutter/flutter#128232

Author: stuartmorgan-g