Require the command in `uvx <name>` to be available in the Python environment (#11603)
Closes https://github.com/astral-sh/uv/issues/7804
Includes a few small minor changes to the messaging, but the primary
change is that in, e.g., `uvx foo`, if the `foo` package does not
provide the `foo` executable we will no longer execute an arbitrary
`foo` executable if present on the `PATH`. This prevents confusing and
surprising behavior, such as the user reported where they did `uv tool
install foobar` (which provides `foo`) then `uvx foo` (which does not
provide `foo`) later falls back to the executable provided by `foobar`
since it's on the `PATH`. We don't enforce this for `--from`, so things
like `uvx --from foo bash -c "..."` are still totally valid. We also
still allow `uvx foo` where the `foo` executable is provided by a
_dependency_ of `foo` instead of `foo` itself.
Most of the diff here is consolidating the logic of the
`hint_on_not_found` and `warn_executable_not_provided_by_package `
utilities.