Require that package descriptions not include newlines (#219)

* Require that package descriptions not include newlines

Previously, we would include the description-with-newlines directly as
the PKG-INFO summary, which could cause subtly broken builds (for
instance, the package may install, but none of the specified
dependencies).

Now, raise a validation error during building, like:

  RuntimeError

  The Poetry configuration is invalid:
    - [description] 'First line\nSecond line (BOOOOOM)' does not match '^[^\n]+$'

Closes python-poetry/poetry#1372

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

@@ -19,7 +19,8 @@
     },
     "description": {
       "type": "string",
-      "description": "Short package description."
+      "description": "Short package description.",
+      "pattern": "^[^\n]*$"
     },
     "keywords": {
       "type": "array",

tests/json/test_poetry_schema.py

@@ -42,3 +42,12 @@ def test_path_dependencies(base_object):
 
 def test_multi_url_dependencies(multi_url_object):
     assert len(validate_object(multi_url_object, "poetry-schema")) == 0
+
+
+def test_multiline_description(base_object):
+    bad_description = "Some multi-\nline string"
+    base_object["description"] = bad_description
+
+    errors = validate_object(base_object, "poetry-schema")
+    assert len(errors) == 1
+    assert errors[0] == "[description] %r does not match '^[^\\n]*$'" % bad_description