Explicitly watch the python version file we read

If we read a python_version_file (specified with the defaults tag
class or the default @@//:.python-version file), explicitly start
watching that file. There are some circumstances where such a watch
wouldn't be allowed, but since it's new functionality it doesn't break
anything for anyone to insist on the watch. If a specific use case
requires the relaxation of the requirement, that can always be
considered later.

Author: vonschultz

python/private/python.bzl

@@ -111,7 +111,7 @@ def parse_modules(*, module_ctx, _fail = fail):
             if default_python_version_file:
                 default_python_version = _one_or_the_same(
                     default_python_version,
-                    module_ctx.read(default_python_version_file).strip(),
+                    module_ctx.read(default_python_version_file, watch = "yes").strip(),
                 )
             if default_python_version_env:
                 default_python_version = module_ctx.getenv(
@@ -121,7 +121,10 @@ def parse_modules(*, module_ctx, _fail = fail):
     if not default_python_version:
         fallback_python_version_file = module_ctx.path(Label("@@//:.python-version"))
         if fallback_python_version_file.exists:
-            default_python_version = module_ctx.read(fallback_python_version_file).strip()
+            default_python_version = module_ctx.read(
+                fallback_python_version_file,
+                watch = "yes",
+            ).strip()
 
     seen_versions = {}
     for mod in module_ctx.modules:

tests/python/python_tests.bzl

@@ -23,7 +23,7 @@ _tests = []
 def _mock_mctx(*modules, environ = {}, mocked_files = {}):
     return struct(
         path = lambda x: struct(exists = x in mocked_files, _file = x),
-        read = lambda x: mocked_files[x._file if "_file" in dir(x) else x],
+        read = lambda x, watch = None: mocked_files[x._file if "_file" in dir(x) else x],
         getenv = environ.get,
         os = struct(environ = environ),
         modules = [