[Fix] respect project root when loading seeds (#8762)

(cherry picked from commit 964e0e4)

.changes/unreleased/Fixes-20231006-134551.yaml

@@ -0,0 +1,6 @@
+kind: Fixes
+body: Enable seeds to be handled from stored manifest data
+time: 2023-10-06T13:45:51.925546-04:00
+custom:
+  Author: michelleark
+  Issue: "6875"

core/dbt/context/providers.py

@@ -769,9 +769,18 @@ def load_agate_table(self) -> agate.Table:
                 'can only load_agate_table for seeds (got a {})'
                 .format(self.model.resource_type)
             )
-        path = os.path.join(
-            self.model.root_path, self.model.original_file_path
+
+        # include package_path for seeds defined in packages
+        package_path = (
+            os.path.join(self.config.packages_install_path, self.model.package_name)
+            if self.model.package_name != self.config.project_name
+            else "."
         )
+        path = os.path.join(self.config.project_root, package_path, self.model.original_file_path)
+        if not os.path.exists(path):
+            assert self.model.root_path
+            path = os.path.join(self.model.root_path, self.model.original_file_path)
+
         column_types = self.model.config.column_types
         try:
             table = agate_helper.from_csv(path, text_columns=column_types)