feat(gazelle): Support "$python_root$" placeholder in the "gazelle:python_visibility" directive

gazelle/README.md

@@ -360,6 +360,19 @@ py_library(
 
 ```
 
+This directive also supports the `$python_root$` placeholder that
+`# gazelle:python_default_visibility` supports.
+
+```starlark
+# gazlle:python_visibility //$python_root$/foo:bar
+
+py_library(
+    ...
+    visibility = ["//this_is_my_python_root/foo:bar"],
+    ...
+)
+```
+
 
 #### Directive: `python_test_file_pattern`:
 

gazelle/python/configure.go

@@ -182,7 +182,8 @@ func (py *Configurer) Configure(c *config.Config, rel string, f *rule.File) {
 				config.SetDefaultVisibility(strings.Split(labels, ","))
 			}
 		case pythonconfig.Visibility:
-			config.AppendVisibility(strings.TrimSpace(d.Value))
+			labels := strings.ReplaceAll(strings.TrimSpace(d.Value), "$python_root$", config.PythonProjectRoot())
+			config.AppendVisibility(labels)
 		case pythonconfig.TestFilePattern:
 			value := strings.TrimSpace(d.Value)
 			if value == "" {

gazelle/python/testdata/directive_python_visibility/subdir_python_root/BUILD.in

@@ -0,0 +1 @@
+# gazelle:python_root

gazelle/python/testdata/directive_python_visibility/subdir_python_root/BUILD.out

@@ -0,0 +1 @@
+# gazelle:python_root

gazelle/python/testdata/directive_python_visibility/subdir_python_root/subdir/BUILD.in

@@ -0,0 +1,6 @@
+# The default visibility is "//$python_root$:__subpackages" so the generated
+# target will also have "//subdir_python_root:__subpackages__" in the visibility
+# attribute.
+#
+# gazelle:python_visibility //$python_root$/anywhere:__pkg__
+# gazelle:python_visibility //$python_root$/and/also:here

gazelle/python/testdata/directive_python_visibility/subdir_python_root/subdir/BUILD.out

@@ -0,0 +1,24 @@
+load("@rules_python//python:defs.bzl", "py_library")
+
+# The default visibility is "//$python_root$:__subpackages" so the generated
+# target will also have "//subdir_python_root:__subpackages__" in the visibility
+# attribute.
+#
+# gazelle:python_visibility //$python_root$/anywhere:__pkg__
+# gazelle:python_visibility //$python_root$/and/also:here
+
+py_library(
+    name = "subdir",
+    srcs = [
+        "__init__.py",
+        "baz.py",
+    ],
+    imports = [".."],
+    visibility = [
+        "//bar:baz",
+        "//subdir_python_root:__subpackages__",
+        "//subdir_python_root/and/also:here",
+        "//subdir_python_root/anywhere:__pkg__",
+        "//tests:__pkg__",
+    ],
+)