Use swh.model.model helpers to compute object identifiers

This will allow us to change the signature of the `swh.model.identifiers`
functions to use model objects directly instead of dicts.

swh/web/browse/snapshot_context.py

@@ -14,15 +14,8 @@ from django.shortcuts import render
 from django.template.defaultfilters import filesizeformat
 from django.utils.html import escape
 
-from swh.model.identifiers import (
-    CONTENT,
-    DIRECTORY,
-    RELEASE,
-    REVISION,
-    SNAPSHOT,
-    snapshot_identifier,
-    swhid,
-)
+from swh.model.identifiers import CONTENT, DIRECTORY, RELEASE, REVISION, SNAPSHOT, swhid
+from swh.model.model import Snapshot
 from swh.web.browse.utils import (
     content_display_max_size,
     format_log_entries,
@@ -59,7 +52,7 @@ from swh.web.common.utils import (
 )
 from swh.web.config import get_config
 
-_empty_snapshot_id = snapshot_identifier({"branches": {}})
+_empty_snapshot_id = Snapshot(branches={}).id.hex()
 
 
 def _get_branch(branches, branch_name, snapshot_id):

swh/web/tests/strategies.py

@@ -23,9 +23,9 @@ from hypothesis.strategies import (
 from swh.model.hashutil import DEFAULT_ALGORITHMS, hash_to_bytes, hash_to_hex
 from swh.model.hypothesis_strategies import origins as new_origin_strategy
 from swh.model.hypothesis_strategies import snapshots as new_snapshot
-from swh.model.identifiers import directory_identifier
 from swh.model.model import (
     Content,
+    Directory,
     Person,
     Revision,
     RevisionType,
@@ -245,7 +245,7 @@ def empty_directory():
     Hypothesis strategy returning the empty directory ingested
     into the test archive.
     """
-    return just(directory_identifier({"entries": []}))
+    return just(Directory(entries=()).id.hex())
 
 
 def unknown_directory():