Begin using Mergeable supplied by dbt-common (#9509)

* Begin using `Mergeable` supplied by `dbt-common`

We're currently in the process of moving the "data resource" portion on nodes
to `dbt/artifacts`. Some of those artifacts depend on `Mergeable` which has
been defined on core. In order to move the data resources to `dbt/artifacts`,
we thus need to move `Mergeable` upstream of core. We moved `Mergeable` to
[dbt-common](https://github.com/dbt-labs/dbt-common) in
dbt-labs/dbt-common#59, and released this change in
[dbt-common 0.1.3](https://pypi.org/project/dbt-common/0.1.3/). As such as, in
order to unblock some of the `dbt/artifacts` migration work, we first need to
update references to `Mergeable` in core to use the `dbt-common` definition.

NOTE: We include changing over to `Replaceable` from `dbt-common` in this
commit. This is because there wasn't a clean way to do it. If I moved the imports
of `Replaceable` on in the files where we updated `Mergeable` then we would
have left `Replaceable` in an inbetween state. If we had moved all instances
of `Replaceable`, it'd be out of scope for the change. As such, it makes more
sense to do that as a separate changeset.

* Remove definition of `Mergeable` from dbt/contracts/util

Although we've removed the definition of `Mergeable` we've ensured the
import paths are still available. We do this because this is under
`contracts`, and the sudden disappearance from the import path might
cause issues for community members using dbt-core as a library.

Ideally we'd define a `Mergeable` class here that inherits the
`dbt-common` definition and raise a deprecation warning on instantiation.
However, we don't have an established strategy to do so.

.changes/unreleased/Under the Hood-20240201-125416.yaml

@@ -0,0 +1,6 @@
+kind: Under the Hood
+body: Start using `Mergeable` from dbt-common
+time: 2024-02-01T12:54:16.462414-08:00
+custom:
+  Author: QMalcolm
+  Issue: "9505"

core/dbt/contracts/graph/unparsed.py

@@ -7,6 +7,7 @@
     AdditionalPropertiesAllowed,
     AdditionalPropertiesMixin,
 )
+from dbt_common.contracts.util import Mergeable
 from dbt_common.exceptions import DbtInternalError, CompilationError
 from dbt_common.dataclass_schema import (
     dbtClassMixin,
@@ -22,10 +23,7 @@
     MaturityType,
     MeasureAggregationParameters,
 )
-from dbt.contracts.util import (
-    Mergeable,
-    Replaceable,
-)
+from dbt.contracts.util import Replaceable
 
 # trigger the PathEncoder
 import dbt_common.helper_types  # noqa:F401

core/dbt/contracts/project.py

@@ -1,7 +1,8 @@
 from dbt import deprecations
-from dbt.contracts.util import Replaceable, Mergeable, list_str, Identifier
+from dbt.contracts.util import Replaceable, list_str, Identifier
 from dbt.adapters.contracts.connection import QueryComment
 from dbt_common.helper_types import NoValue
+from dbt_common.contracts.util import Mergeable
 from dbt_common.dataclass_schema import (
     dbtClassMixin,
     ValidationError,

core/dbt/contracts/util.py

@@ -1,8 +1,9 @@
-import dataclasses
 from typing import List, Any, Tuple
 
 from dbt_common.dataclass_schema import ValidatedStringMixin, ValidationError
-from dbt_common.contracts.util import Replaceable
+
+# Leave imports of `Mergeable` and `Replaceable` to preserve import paths
+from dbt_common.contracts.util import Mergeable, Replaceable  # noqa:F401
 
 
 SourceKey = Tuple[str, str]
@@ -24,22 +25,6 @@ class Foo:
     return []
 
 
-class Mergeable(Replaceable):
-    def merged(self, *args):
-        """Perform a shallow merge, where the last non-None write wins. This is
-        intended to merge dataclasses that are a collection of optional values.
-        """
-        replacements = {}
-        cls = type(self)
-        for arg in args:
-            for field in dataclasses.fields(cls):
-                value = getattr(arg, field.name)
-                if value is not None:
-                    replacements[field.name] = value
-
-        return self.replace(**replacements)
-
-
 class Identifier(ValidatedStringMixin):
     """Our definition of a valid Identifier is the same as what's valid for an unquoted database table name.