store: only access file_size information through the file object

This make sure other code only access this information through the proper API,
and it prepare for store entries to be able to agregate multiple files.

mercurial/store.py

@@ -461,7 +461,7 b' class BaseStoreEntry:'
 
     unencoded_path = attr.ib()
     is_volatile = attr.ib(default=False)
-    file_size = attr.ib(default=None)
+    _file_size = attr.ib(default=None)
 
     def __init__(
         self,
@@ -471,13 +471,13 b' class BaseStoreEntry:'
     ):
         self.unencoded_path = unencoded_path
         self.is_volatile = is_volatile
-        self.file_size = file_size
+        self._file_size = file_size
 
     def files(self):
         return [
             StoreFile(
                 unencoded_path=self.unencoded_path,
-                file_size=self.file_size,
+                file_size=self._file_size,
                 is_volatile=self.is_volatile,
             )
         ]