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.
```

scripts/uvx_dummy/README.md

@@ -0,0 +1,8 @@
+# uvx
+
+`uvx` is provided by the [uv package](https://pypi.org/project/uv/). There is no need to install it
+separately. This is just a dummy package guarding against dependendency confusion attacks.
+
+Previously, this was a third-party package used to extend uv's functionality. The author of that
+package graciously renamed it to [`uvenv`](https://pypi.org/project/uvenv/) to avoid confusion. If
+you're attempting to use that package, replace your dependency on `uvx` with `uvenv`.

scripts/uvx_dummy/build_backend.py

@@ -0,0 +1,21 @@
+import sys
+
+from hatchling.build import *  # noqa:F401,F403
+
+
+def build_wheel(wheel_directory, config_settings, metadata_directory) -> str:
+    print(
+        "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.",
+        file=sys.stderr,
+    )
+    sys.exit(1)
+
+
+def prepare_metadata_for_build_wheel(metadata_directory, config_settings) -> str:
+    print(
+        "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.",
+        file=sys.stderr,
+    )
+    sys.exit(1)

scripts/uvx_dummy/pyproject.toml

@@ -0,0 +1,12 @@
+[project]
+name = "uvx"
+version = "3.0.0"
+description = "A placeholder package to prevent dependency confusion with `uvx`."
+authors = [{ name = "Astral Software Inc.", email = "hey@astral.sh" }]
+requires-python = ">=3.8"
+readme = "README.md"
+
+[build-system]
+backend-path = ["."]
+requires = ["hatchling"]
+build-backend = "build_backend"