build: do not generate setup.py by default

This is the first step in removing setup file generation from Poetry
projects. With this change, projects that require a setup.py to be
generated when a build script is used needs to explicitly set
`tool.poetry.build.generate-setup-file`, introduced in #26, to `true`
in `pyproject.toml`.

src/poetry/core/json/schemas/poetry-schema.json

@@ -666,7 +666,7 @@
         "generate-setup-file": {
           "type": "boolean",
           "description": "Generate and include a setup.py file in sdist.",
-          "default": true
+          "default": false
         },
         "script": {
           "$ref": "#/definitions/build-script"

src/poetry/core/packages/project_package.py

@@ -72,4 +72,4 @@ def urls(self) -> dict[str, Any]:
         return urls
 
     def build_should_generate_setup(self) -> bool:
-        return self.build_config.get("generate-setup-file", True)
+        return self.build_config.get("generate-setup-file", False)