Add managed_directories attribute to workspace() function.

This is only a part of the incrementally updated user-owned directory feature; that is why the parsed value is not yet used in computations.

- Under --experimental_allow_incremental_repository_updates flag.
- Parse results are put into WorkspaceFileValue map field.

PiperOrigin-RevId: 244819268
diff --git a/src/main/java/com/google/devtools/build/lib/syntax/StarlarkSemantics.java b/src/main/java/com/google/devtools/build/lib/syntax/StarlarkSemantics.java
index fc5ab5d..bc025a3 100644
--- a/src/main/java/com/google/devtools/build/lib/syntax/StarlarkSemantics.java
+++ b/src/main/java/com/google/devtools/build/lib/syntax/StarlarkSemantics.java
@@ -39,6 +39,8 @@
    * the exact name of the flag transformed to upper case (for error representation).
    */
   public enum FlagIdentifier {
+    EXPERIMENTAL_ALLOW_INCREMENTAL_REPOSITORY_UPDATES(
+        StarlarkSemantics::experimentalAllowIncrementalRepositoryUpdates),
     EXPERIMENTAL_ENABLE_ANDROID_MIGRATION_APIS(
         StarlarkSemantics::experimentalEnableAndroidMigrationApis),
     EXPERIMENTAL_BUILD_SETTING_API(StarlarkSemantics::experimentalBuildSettingApi),
@@ -114,6 +116,8 @@
       AutoValue_StarlarkSemantics.class;
 
   // <== Add new options here in alphabetic order ==>
+  public abstract boolean experimentalAllowIncrementalRepositoryUpdates();
+
   public abstract boolean experimentalBuildSettingApi();
 
   public abstract ImmutableList<String> experimentalCcSkylarkApiEnabledPackages();
@@ -209,6 +213,7 @@
           // <== Add new options here in alphabetic order ==>
           .experimentalBuildSettingApi(false)
           .experimentalCcSkylarkApiEnabledPackages(ImmutableList.of())
+          .experimentalAllowIncrementalRepositoryUpdates(false)
           .experimentalEnableAndroidMigrationApis(false)
           .experimentalGoogleLegacyApi(false)
           .experimentalJavaCommonCreateProviderEnabledPackages(ImmutableList.of())
@@ -253,6 +258,8 @@
   public abstract static class Builder {
 
     // <== Add new options here in alphabetic order ==>
+    public abstract Builder experimentalAllowIncrementalRepositoryUpdates(boolean value);
+
     public abstract Builder experimentalBuildSettingApi(boolean value);
 
     public abstract Builder experimentalCcSkylarkApiEnabledPackages(List<String> value);