fix: also validate existing dependency groups

Signed-off-by: Henry Schreiner <henryschreineriii@gmail.com>

src/validate_pyproject/extra_validations.py

@@ -24,6 +24,13 @@ class RedefiningStaticFieldAsDynamic(ValidationError):
     )
 
 
+class IncludedDependencyGroupMustExist(ValidationError):
+    _DESC = """An included dependency group must exist and must not be cyclic.
+    """
+    __doc__ = _DESC
+    _URL = "https://peps.python.org/pep-0735/"
+
+
 def validate_project_dynamic(pyproject: T) -> T:
     project_table = pyproject.get("project", {})
     dynamic = project_table.get("dynamic", [])
@@ -49,4 +56,28 @@ def validate_project_dynamic(pyproject: T) -> T:
     return pyproject
 
 
-EXTRA_VALIDATIONS = (validate_project_dynamic,)
+def validate_include_depenency(pyproject: T) -> T:
+    dependency_groups = pyproject.get("dependency-groups", {})
+    for key, value in dependency_groups.items():
+        for each in value:
+            if (
+                isinstance(each, dict)
+                and (include_group := each.get("include-group"))
+                and include_group not in dependency_groups
+            ):
+                raise IncludedDependencyGroupMustExist(
+                    message=f"The included dependency group {include_group} doesn't exist",
+                    value={
+                        "field": include_group,
+                    },
+                    name=f"data.dependency_groups.{key}",
+                    definition={
+                        "description": cleandoc(IncludedDependencyGroupMustExist._DESC),
+                        "see": IncludedDependencyGroupMustExist._URL,
+                    },
+                    rule="PEP 621",
+                )
+    return pyproject
+
+
+EXTRA_VALIDATIONS = (validate_project_dynamic, validate_include_depenency)