Clean up '#if CORE' - use SecurityZone from CoreFX (#4368)

Make `System.Management.Automation references` to package `System.Security.Permissions`, which brings back `SecurityZone` and `System.Security.Policy.Zone`.

src/System.Management.Automation/CoreCLR/CorePsStub.cs

@@ -16,24 +16,6 @@ using Microsoft.Win32;
 // We use the stubs in this namespace to reduce #if/def in the code as much as possible.
 namespace Microsoft.PowerShell.CoreClr.Stubs
 {
-    #region Misc_Types
-
-    /// <summary>
-    /// Stub for SecurityZone
-    /// </summary>
-    public enum SecurityZone
-    {
-        MyComputer = 0,
-        Intranet = 1,
-        Trusted = 2,
-        Internet = 3,
-        Untrusted = 4,
-
-        NoZone = -1,
-    }
-
-    #endregion Misc_Types
-
     #region SystemManagementStubs
 
     // Summary:

src/System.Management.Automation/System.Management.Automation.csproj

@@ -16,6 +16,7 @@
     <PackageReference Include="System.IO.FileSystem.AccessControl" Version="4.4.0-preview3-25423-02" />
     <PackageReference Include="System.Security.AccessControl" Version="4.4.0-preview3-25423-02" />
     <PackageReference Include="System.Security.Cryptography.Pkcs" Version="4.4.0-preview3-25423-02" />
+    <PackageReference Include="System.Security.Permissions" Version="4.4.0-preview3-25423-02" />
     <PackageReference Include="System.Text.Encoding.CodePages" Version="4.4.0-preview3-25423-02" />
     <PackageReference Include="Microsoft.Management.Infrastructure" Version="1.0.0-alpha*" />
   </ItemGroup>

src/System.Management.Automation/engine/Modules/ImportModuleCommand.cs

@@ -27,11 +27,6 @@ using Token = System.Management.Automation.Language.Token;
 using Microsoft.PowerShell.Telemetry.Internal;
 #endif
 
-#if CORECLR
-// Use stub for SecurityZone.
-using Microsoft.PowerShell.CoreClr.Stubs;
-#endif
-
 //
 // Now define the set of commands for manipulating modules.
 //

src/System.Management.Automation/namespaces/FileSystemProvider.cs

@@ -24,11 +24,6 @@ using Dbg = System.Management.Automation;
 using System.Runtime.InteropServices;
 using System.Management.Automation.Runspaces;
 
-#if CORECLR
-// Use stubs for SecurityZone
-using Microsoft.PowerShell.CoreClr.Stubs;
-#endif
-
 namespace Microsoft.PowerShell.Commands
 {
     #region FileSystemProvider

src/System.Management.Automation/security/SecurityManager.cs

@@ -14,11 +14,6 @@ using System.Management.Automation.Language;
 using System.Security;
 using System.Security.Cryptography.X509Certificates;
 
-#if CORECLR
-// Use stub for SecurityZone
-using Microsoft.PowerShell.CoreClr.Stubs;
-#endif
-
 namespace Microsoft.PowerShell
 {
     /// <summary>

src/System.Management.Automation/utils/ClrFacade.cs

@@ -21,7 +21,6 @@ using System.Runtime.InteropServices.ComTypes;
 #if CORECLR
 using System.Runtime.Loader; /* used in facade APIs related to assembly operations */
 using System.Management.Automation.Host;          /* used in facade API 'GetUninitializedObject' */
-using Microsoft.PowerShell.CoreClr.Stubs;         /* used in facade API 'GetFileSecurityZone' */
 using System.Management.Automation.Internal;
 using System.Text.RegularExpressions;
 #else