Wrap non-seekable stream in XamlToRtfWriter

[stevenbrix]

When we started using System.IO.Packaging that came in corefx as part of this commit 71c9337d, the behavior of the streams that were returned from PackagePart.GetStream are now non-seekable. The behavior change of this issue is described in #585, with links to the corresponding corefx issue that discusses the behavior change. The workaround for the behavior change is to wrap the stream in a MemoryStream to allow seek-ability.

Due to the refactor, the zip implementation inside of WindowsBase is no longer needed. I've removed it as part of this change since this should have been removed with the original refactor, but was missed.

The PackUriHelper.GetPartUri method was also missing in CoreFx. This was added with dotnet/corefx#38699. I've updated the code to remove more of our custom duplicated logic in favor for what is in corefx. No code that is in our internal branch depends on these APIs, they already use the PackUriHelper APIs that come from System.IO.Packaging.

Fixes #858
Fixes #597

[vatsan-madhavan]

lgtm.

the previously failing tests pass now, do they?

[vatsan-madhavan]

remove comment

[stevenbrix]

lgtm.

the previously failing tests pass now, do they?

@vatsan-madhavan, yeah the tests pass. Some of theme are no longer needed (like DrtZipIO) and quite a few failures were also just due to test code needing to be updated. Most of the tests use reflection to discover and invoke methods and that is the biggest reason they've started failing (rather than breaking at compile time). They were either trying to reflect and invoke internal methods that were removed, or the parameters were slightly different (removal of the bool streaming parameter from Package.Open). Most tests don't even rely on the streaming functionality that existed. This is at least true for the DRTs, I'm not sure what subset of feature tests that might be broken, but fixing those should be fairly straightforward as well.