Add utility for checking file existence.

Avoids continued copy-paste of this like https://github.com/aspect-build/rules_ts/blob/main/ts/defs.bzl#L35-L44

docs/utils.md

@@ -2,6 +2,28 @@
 
 Public API
 
+<a id="file_exists"></a>
+
+## file_exists
+
+<pre>
+file_exists(<a href="#file_exists-path">path</a>)
+</pre>
+
+Check whether a file exists.
+
+Useful in macros to set defaults for a configuration file if it is present.
+This can only be called during the loading phase, not from a rule implementation.
+
+
+**PARAMETERS**
+
+
+| Name  | Description | Default Value |
+| :------------- | :------------- | :------------- |
+| <a id="file_exists-path"></a>path |  a label, or a string which is a path relative to this package   |  none |
+
+
 <a id="glob_directories"></a>
 
 ## glob_directories
@@ -51,7 +73,7 @@ a bool
 path_to_workspace_root()
 </pre>
 
- Retuns the path to the workspace root under bazel
+ Returns the path to the workspace root under bazel
 
 
 **RETURNS**

lib/private/utils.bzl

@@ -84,7 +84,7 @@ def _is_external_label(param):
 
 # Path to the root of the workspace
 def _path_to_workspace_root():
-    """ Retuns the path to the workspace root under bazel
+    """ Returns the path to the workspace root under bazel
 
     Returns:
         Path to the workspace root
@@ -98,8 +98,24 @@ def _glob_directories(include, **kwargs):
     directories = [p for p in all if p not in files]
     return directories
 
+def _file_exists(path):
+    """Check whether a file exists.
+
+    Useful in macros to set defaults for a configuration file if it is present.
+    This can only be called during the loading phase, not from a rule implementation.
+
+    Args:
+        path: a label, or a string which is a path relative to this package
+    """
+    label = _to_label(path)
+    file_abs = "%s/%s" % (label.package, label.name)
+    file_rel = file_abs[len(native.package_name()) + 1:]
+    file_glob = native.glob([file_rel])
+    return len(file_glob) > 0
+
 utils = struct(
     is_external_label = _is_external_label,
+    file_exists = _file_exists,
     glob_directories = _glob_directories,
     path_to_workspace_root = _path_to_workspace_root,
     propagate_well_known_tags = _propagate_well_known_tags,

lib/tests/utils_test.bzl

@@ -75,6 +75,7 @@ propagate_well_known_tags_test = unittest.make(
     _propagate_well_known_tags_test_impl,
 )
 
+# buildifier: disable=function-docstring
 def utils_test_suite():
     to_label_test(name = "to_label_tests", relative_asserts = {
         utils.to_label(":utils_test.bzl"): "//lib/tests:utils_test.bzl",
@@ -89,3 +90,9 @@ def utils_test_suite():
     propagate_well_known_tags_test(
         name = "propagate_well_known_tags_tests",
     )
+
+    # Tests that must run in the loading phase
+    if utils.file_exists("does-not-exist"):
+        fail("does-not-exist does not exist")
+    if not utils.file_exists("utils_test.bzl"):
+        fail("utils_test.bzl does exist")

lib/utils.bzl

@@ -7,3 +7,4 @@ glob_directories = utils.glob_directories
 path_to_workspace_root = utils.path_to_workspace_root
 propagate_well_known_tags = utils.propagate_well_known_tags
 to_label = utils.to_label
+file_exists = utils.file_exists