Preprocessed output of a new project that imports a file with an xmlns is littered with xmlns

When the entry-point project doesn't specify an `xmlns` (as new "clean" templates don't), but it imports a file that uses the `xmlns` (like, say, `Microsoft.Common.targets`), the resultant preprocessed output puts an `xmlns=` attribute on _every_ known-to-MSBuild element in the output.

I noticed this on a new-web-template project where it's amazingly verbose, but here's a minimal repro:

```
S:\work\xmlns>type *proj

imported.proj

project.proj

S:\work\xmlns>S:\msbuild\bin\Bootstrap\15.0\Bin\MSBuild.exe /pp project.proj

```