[external-assets] Rename FromSourceAsset -> FromLoadableAsset (#20546)

## Summary & Motivation

Rename `FromSourceAsset` (a subclass of `StepInputSource`) to
`FromLoadableAsset`. This is clearer and more consistent with new
terminology in the codebase.

## How I Tested These Changes

Existing test suite.

python_modules/dagster/dagster/_core/execution/plan/inputs.py

@@ -122,10 +122,12 @@ def compute_version(
         raise NotImplementedError()
 
 
-@whitelist_for_serdes(storage_field_names={"node_handle": "solid_handle"})
-class FromSourceAsset(
+@whitelist_for_serdes(
+    storage_name="FromSourceAsset", storage_field_names={"node_handle": "solid_handle"}
+)
+class FromLoadableAsset(
     NamedTuple(
-        "_FromSourceAsset",
+        "_FromLoadableAsset",
         [
             ("node_handle", NodeHandle),
             ("input_name", str),

python_modules/dagster/dagster/_core/execution/plan/plan.py

@@ -62,10 +62,10 @@
     FromDirectInputValue,
     FromDynamicCollect,
     FromInputManager,
+    FromLoadableAsset,
     FromMultipleSources,
     FromMultipleSourcesLoadSingleSource,
     FromPendingDynamicStepOutput,
-    FromSourceAsset,
     FromStepOutput,
     FromUnresolvedStepOutput,
     StepInput,
@@ -441,7 +441,7 @@ def get_step_input_source(
     ):
         # can only load from source asset if assets defs are available
         if asset_layer.asset_key_for_input(handle, input_handle.input_name):
-            return FromSourceAsset(node_handle=handle, input_name=input_name)
+            return FromLoadableAsset(node_handle=handle, input_name=input_name)
         elif input_def.input_manager_key:
             return FromInputManager(node_handle=handle, input_name=input_name)
 
@@ -626,7 +626,7 @@ def _step_input_source_from_blocking_asset_checks_dep_def(
     if source_to_load_from is None:
         asset_key_for_input = asset_layer.asset_key_for_input(node_handle, input_handle.input_name)
         if asset_key_for_input:
-            source_to_load_from = FromSourceAsset(node_handle=node_handle, input_name=input_name)
+            source_to_load_from = FromLoadableAsset(node_handle=node_handle, input_name=input_name)
             sources.append(source_to_load_from)
         else:
             check.failed("Unexpected: no sources to load from and no asset key to load from")