build.psm1 changes for Activities dlls

- Add Start-PSBuild -Clean
- Normalize $env:PATH references in .\build.psm1
- Add more logging in Resolve-Xaml for troubleshooting
- Always use msbuild from the Microsoft.NET v4.0.30319
- Rename Resolve-Xaml into Start-XamlGen for consistency with Start-TypeGen and Start-ResGen
- Disable pwrshplugin build temporarely

src/Microsoft.PowerShell.ConsoleHost/project.json

@@ -25,6 +25,9 @@
             "../Modules",
             "powershell.exe",
             "../../powershell.version"
+        ],
+        "exclude": [
+            "../Modules/Pester/.git"
         ]
     },