Add uvx placeholder package
This package is placeholder that blocks `[uv] pip install uvx` and redirects the user to uv proper instead. It does so through a source distribution that fails to build into a wheel.
```
$ uv pip install dist/uvx-3.0.0.tar.gz
Using Python 3.13.2 environment at: /home/konsti/projects/uv/.venv
Resolved 1 package in 1ms
× Failed to build `uvx @ file:///home/konsti/projects/uv/scripts/uvx_dummy/dist/uvx-3.0.0.tar.gz`
├─▶ The build backend returned an error
╰─▶ Call to `build_backend.build_wheel` failed (exit status: 1)
[stderr]
This package is a placeholder to prevent dependency confusion with `uvx`. Please refer to https://github.com/astral-sh/uv for installing
uv and uvx.
hint: This usually indicates a problem with the package or the build environment.
```
```
$ pip install dist/uvx-3.0.0.tar.gz
Processing ./dist/uvx-3.0.0.tar.gz
Installing build dependencies ... done
Getting requirements to build wheel ... done
Preparing metadata (pyproject.toml) ... error
error: subprocess-exited-with-error
× Preparing metadata (pyproject.toml) did not run successfully.
│ exit code: 1
╰─> [1 lines of output]
This package is a placeholder to prevent dependency confusion with `uvx`. Please refer to https://github.com/astral-sh/uv for installing uv and uvx.
[end of output]
note: This error originates from a subprocess, and is likely not a problem with pip.
error: metadata-generation-failed
× Encountered error while generating package metadata.
╰─> See above for output.
note: This is an issue with the package mentioned above, not pip.
hint: See above for details.
```
