Set isolated when run with -I (#1426)

Author: slozier

Src/IronPython/Hosting/PythonCommandLine.cs

@@ -193,7 +193,9 @@ protected override void Initialize() {
                 }
             }
 
-            PythonContext.InsertIntoPath(0, fullPath);
+            if (!PythonContext.PythonOptions.Isolated) {
+                PythonContext.InsertIntoPath(0, fullPath);
+            }
             PythonContext.MainThread = Thread.CurrentThread;
         }
 

Src/IronPython/Hosting/PythonOptionsParser.cs

@@ -79,10 +79,11 @@ private ReadOnlySpan<char> HandleOptions(ReadOnlySpan<char> options) {
                     LanguageSetup.Options["Verbose"] = ScriptingRuntimeHelpers.True;
                     break;
 
-                case 'I': // both -E and -s
+                case 'I': // also implies both -E and -s
                     ConsoleOptions.IgnoreEnvironmentVariables = true;
                     LanguageSetup.Options["IgnoreEnvironment"] = ScriptingRuntimeHelpers.True;
                     LanguageSetup.Options["NoUserSite"] = ScriptingRuntimeHelpers.True;
+                    LanguageSetup.Options["Isolated"] = ScriptingRuntimeHelpers.True;
                     break;
 
                 case 'S':

Src/IronPython/Runtime/PythonContext.cs

@@ -728,6 +728,7 @@ private void InitializeSysFlags() {
                 _ => (int)PythonOptions.BytesWarning
             };
             flags.quiet = PythonOptions.Quiet ? 1 : 0;
+            flags.isolated = PythonOptions.Isolated ? 1 : 0;
         }
 
         internal bool ShouldInterpret(PythonCompilerOptions options, SourceUnit source) {

Src/IronPython/Runtime/PythonOptions.cs

@@ -63,6 +63,11 @@ public sealed class PythonOptions : LanguageOptions {
         /// </summary>
         public bool IgnoreEnvironment { get; }
 
+        /// <summary>
+        /// Run in isolated mode.
+        /// </summary>
+        public bool Isolated { get; }
+
         /// <summary>
         /// Enables the verbose option which traces import statements.  This is ignored by IronPython
         /// except for setting sys.flags.
@@ -146,6 +151,7 @@ public PythonOptions(IDictionary<string, object> options)
             NoDebug = GetOption(options, "NoDebug", (Regex)null);
             Quiet = GetOption(options, "Quiet", false);
             NoImportLib = GetOption(options, "NoImportLib", false);
+            Isolated = GetOption(options, "Isolated", false);
         }
 
         private static IDictionary<string, object> EnsureSearchPaths(IDictionary<string, object> options) {