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/main_tests/main_options_test.py

@@ -11,6 +11,7 @@ from cibuildwheel.frontend import _split_config_settings
 from cibuildwheel.options import BuildOptions, _get_pinned_container_images
 from cibuildwheel.selector import BuildSelector, EnableGroup
 from cibuildwheel.util import resources
+from cibuildwheel.util.packaging import DependencyConstraints
 
 # CIBW_PLATFORM is tested in main_platform_test.py
 
@@ -342,6 +343,43 @@ def test_before_all(before_all, platform_specific, platform, intercepted_build_a
     assert build_options.before_all == (before_all or "")
 
 
+@pytest.mark.parametrize(
+    "dependency_versions",
+    [None, "pinned", "latest", "FILE", "packages: pip==21.0.0"],
+)
+@pytest.mark.parametrize("platform_specific", [False, True])
+def test_dependency_versions(
+    dependency_versions, platform_specific, platform, intercepted_build_args, monkeypatch, tmp_path
+):
+    option_value = dependency_versions
+
+    if dependency_versions == "FILE":
+        constraints_file = tmp_path / "constraints.txt"
+        constraints_file.write_text("foo==1.2.3\nbar==4.5.6")
+        option_value = str(constraints_file)
+
+    if option_value is not None:
+        if platform_specific:
+            monkeypatch.setenv("CIBW_DEPENDENCY_VERSIONS_" + platform.upper(), option_value)
+            monkeypatch.setenv("CIBW_DEPENDENCY_VERSIONS", "overwritten")
+        else:
+            monkeypatch.setenv("CIBW_DEPENDENCY_VERSIONS", option_value)
+
+    main()
+
+    build_options: BuildOptions = intercepted_build_args.args[0].build_options(identifier=None)
+    dependency_constraints = build_options.dependency_constraints
+    if dependency_versions is None or dependency_versions == "pinned":
+        assert dependency_constraints == DependencyConstraints.pinned()
+    elif dependency_versions == "latest":
+        assert dependency_constraints == DependencyConstraints.latest()
+    elif dependency_versions == "FILE":
+        assert dependency_constraints.base_file_path
+        assert dependency_constraints.base_file_path.samefile(Path(option_value))
+    elif dependency_versions.startswith("packages:"):
+        assert dependency_constraints.packages == ["pip==21.0.0"]
+
+
 @pytest.mark.parametrize("method", ["unset", "command_line", "env_var"])
 def test_debug_traceback(monkeypatch, method, capfd):
     if method == "command_line":