Add support for git submodules in Get-Extension

Author: shivammathur

extension/BuildPhpExtension/private/Get-Extension.ps1

@@ -77,6 +77,11 @@ function Get-Extension {
                         git remote add origin $ExtensionUrl > $null 2>&1
                         git fetch --depth=1 origin $ExtensionRef > $null 2>&1
                         git checkout FETCH_HEAD > $null 2>&1
+
+                        if (Test-Path -LiteralPath (Join-Path (Get-Location).Path '.gitmodules')) {
+                            git submodule sync --recursive > $null 2>&1
+                            git submodule update --init --recursive --depth 1 > $null 2>&1
+                        }
                     }
                 }
             } catch {