cache_sync directory: add a dummy __init__.py to get rid of setuptools warning

src/borg/cache_sync/__init__.py

@@ -0,0 +1,23 @@
+"""
+dummy package init file to suppress this weird setuptools warning:
+
+$ pip install -v .  # note: does not happen with -ve .
+
+############################
+# Package would be ignored #
+############################
+Python recognizes 'borg.cache_sync' as an importable package,
+but it is not listed in the `packages` configuration of setuptools.
+
+'borg.cache_sync' has been automatically added to the distribution only
+because it may contain data files, but this behavior is likely to change
+in future versions of setuptools (and therefore is considered deprecated).
+
+Please make sure that 'borg.cache_sync' is included as a package by using
+the `packages` configuration field or the proper discovery methods
+(for example by using `find_namespace_packages(...)`/`find_namespace:`
+instead of `find_packages(...)`/`find:`).
+
+You can read more about "package discovery" and "data files" on setuptools
+documentation page.
+"""