Added support for recursive copying from output\ directory in package.

This change allows for creation of directory subtrees inside the output folder. 

Example:
```XML
<file src="output\NugetToOutputFolder.dll" target="output\NugetToOutputFolder.dll" />
<file src="output\SecondOutputFolder.dll" target="output\test\2nd.dll"/>
<file src="Third\bin\Debug\Third.dll" target="output\3rdtest\deeper-recursive\3rd.dll"/>
```

Output directory:
```
bin
 |- Debug
       |- NugetToOutputFolder.dll
       |- test
       |    |- 2nd.dll
       |- 3rdtest
               |- deeper-recursive
                               |- 3rd.dll
```