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.

.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,