Inline dependency-versions syntax (#2122)

* Write docs for inline dependency-versions

* Change the `inline` keyword to `packages` for better readability

* Implement inline package constraints

* [pre-commit.ci] auto fixes from pre-commit.com hooks

for more information, see https://pre-commit.ci

* Add schema for TOML properties

* Change the parsing of the option to parse filenames as-is

* Add a unit test for table-parsing of the option

* Remove unneeded shlex.quote on the dependency-version test

* Tidy-ups, comments, docs fixes

* Add test for empty packages option value

* Fix empty packages scenario

And, remove some optionals to reduce the problem space

---------

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

unit_test/dependency_constraints_test.py

@@ -1,21 +1,55 @@
 from pathlib import Path
 
+import pytest
+
 from cibuildwheel.util.packaging import DependencyConstraints
 
 
-def test_defaults():
-    dependency_constraints = DependencyConstraints.with_defaults()
+def test_defaults(tmp_path: Path) -> None:
+    dependency_constraints = DependencyConstraints.pinned()
 
     project_root = Path(__file__).parents[1]
     resources_dir = project_root / "cibuildwheel" / "resources"
 
+    assert dependency_constraints.base_file_path
     assert dependency_constraints.base_file_path.samefile(resources_dir / "constraints.txt")
-    assert dependency_constraints.get_for_python_version("3.99").samefile(
-        resources_dir / "constraints.txt"
+
+    constraints_file = dependency_constraints.get_for_python_version(
+        version="3.99", tmp_dir=tmp_path
     )
-    assert dependency_constraints.get_for_python_version("3.9").samefile(
-        resources_dir / "constraints-python39.txt"
+    assert constraints_file
+    assert constraints_file.samefile(resources_dir / "constraints.txt")
+
+    constraints_file = dependency_constraints.get_for_python_version(
+        version="3.9", tmp_dir=tmp_path
     )
-    assert dependency_constraints.get_for_python_version("3.13").samefile(
-        resources_dir / "constraints-python313.txt"
+    assert constraints_file
+    assert constraints_file.samefile(resources_dir / "constraints-python39.txt")
+
+    constraints_file = dependency_constraints.get_for_python_version(
+        version="3.13", tmp_dir=tmp_path
     )
+    assert constraints_file
+    assert constraints_file.samefile(resources_dir / "constraints-python313.txt")
+
+
+def test_inline_packages(tmp_path: Path) -> None:
+    dependency_constraints = DependencyConstraints(
+        base_file_path=None,
+        packages=["foo==1.2.3", "bar==4.5.6"],
+    )
+
+    constraint_file = dependency_constraints.get_for_python_version(version="x.x", tmp_dir=tmp_path)
+    assert constraint_file
+    constraints_file_contents = constraint_file.read_text()
+
+    assert constraints_file_contents == "foo==1.2.3\nbar==4.5.6"
+
+
+@pytest.mark.parametrize("config_string", ["", "latest", "packages:"])
+def test_empty_constraints(config_string: str) -> None:
+    dependency_constraints = DependencyConstraints.from_config_string(config_string)
+
+    assert not dependency_constraints.packages
+    assert not dependency_constraints.base_file_path
+    assert dependency_constraints == DependencyConstraints.latest()