Add support for generating the C# SDK assembly using CSC.exe on Windows PS. (#312)

Latest version of AutoRest generates C# 6 code. Add-Type on Windows PS doesn't support compilation of C# 6 code.
Made required changes to leverage the CSC.exe for generating the C# SDK assembly on Windows PowerShell.

Additional changes
- Updated readme and run-tests.ps1
- Removed stale source code files under PSSwagger.Azure.Helpers folder.
- Removed `Import-Module '$PSScriptRoot\PSSwaggerUtility'` usage for PowerShellCore.

Author: bmanikm

PSSwagger/PSSwagger.Azure.Helpers/PSSwagger.Azure.Helpers.psd1

@@ -25,6 +25,7 @@ ConvertFrom-StringData @'
     SamePropertyName=Same property name '{0}' is defined in a definition '{1}' with AllOf inheritance.
     DataTypeNotImplemented=Please get an implementation of '{0}' for '{1}'
     AutoRestNotInPath=Unable to find AutoRest in PATH environment. Ensure the PATH is updated.
+    CscExeNotInPath=Unable to find CSC.exe in PATH environment. Ensure the PATH is updated with CSC.exe location.
     AutoRestError=AutoRest resulted in an error
     SwaggerParamsMissing=No parameters in the Swagger
     SwaggerDefinitionsMissing=No definitions in the Swagger

PSSwagger/PSSwagger.Azure.Helpers/PSSwagger.Azure.Helpers.psm1

@@ -587,7 +587,12 @@ function ConvertTo-CsharpCode
 
     $autoRestExePath = "AutoRest"
     if (-not (get-command -name $autoRestExePath)) {
-            throw $LocalizedData.AutoRestNotInPath
+        throw $LocalizedData.AutoRestNotInPath
+    }
+
+    if (-not (Get-OperatingSystemInfo).IsCore -and 
+        (-not (Get-Command -Name 'Csc.Exe' -ErrorAction Ignore))) {
+        throw $LocalizedData.CscExeNotInPath
     }
 
     $outputDirectory = $SwaggerMetaDict['outputDirectory']
@@ -718,7 +723,7 @@ function ConvertTo-CsharpCode
                                                 -CliXmlTmpPath $cliXmlTmpPath"
 
         $success = & "powershell" -command "& {$command}"
-        
+
         $codeReflectionResult = Import-CliXml -Path $cliXmlTmpPath
         if ($codeReflectionResult.ContainsKey('VerboseMessages') -and $codeReflectionResult.VerboseMessages -and ($codeReflectionResult.VerboseMessages.Count -gt 0)) {
             $verboseMessages = $codeReflectionResult.VerboseMessages -Join [Environment]::NewLine
@@ -773,8 +778,7 @@ function ConvertTo-CsharpCode
         }
         $dependencies = Get-PSSwaggerExternalDependencies -Azure:$codeCreatedByAzureGenerator -Framework 'netstandard1'
         $microsoftRestClientRuntimeAzureRequiredVersion = if ($dependencies.ContainsKey('Microsoft.Rest.ClientRuntime.Azure')) { $dependencies['Microsoft.Rest.ClientRuntime.Azure'].RequiredVersion } else { '' }
-        $command = "Import-Module '$PSScriptRoot\PSSwaggerUtility';
-                    Add-PSSwaggerClientType -OutputAssemblyName '$outAssembly' ``
+        $command = "PSSwaggerUtility\Add-PSSwaggerClientType -OutputAssemblyName '$outAssembly' ``
                                                -ClrPath '$clrPath' ``
                                                -CSharpFiles $allCSharpFilesArrayString ``
                                                -MicrosoftRestClientRuntimeAzureRequiredVersion '$microsoftRestClientRuntimeAzureRequiredVersion' ``