feat: add syntax sugar to avoid meaningless requirements.in files (#134)

This is a minor quality-of-life improvement for some cases, like
bazelbuild/rules_python#1016 where you just
need a requirements file for the `twine` package so it's nice to write
it as

```
uv_pip_compile(
    name = "twine_requirements",
    requirements_in = ["twine"],
    requirements_txt = "requirements.twine",
)
```

examples/typical/BUILD.bazel

@@ -9,6 +9,12 @@ pip_compile(
     requirements_txt = "requirements_linux.txt",
 )
 
+pip_compile(
+    name = "generate_requirements_twine",
+    requirements_in = ["twine"],
+    requirements_txt = "requirements_twine.txt",
+)
+
 create_venv(name = "create-venv")
 
 create_venv(