Missing transitive dependencies after paket update #1190

Reproduction case:

1. Create a new project targeting .NET 4.5.
2. Run paket.exe add nuget xunit project myproject.tests
3. Note that xunit.abstractions is not in the list of references

This can also be reproduced by taking an existing project with current xunit references and then running a paket update.

The relevant diff after performing an update in one of my own projects:

--- i/paket.lock
+++ w/paket.lock
@@ -134,7 +134,7 @@ NUGET
     xunit (2.1.0)
       xunit.assert (2.1.0)
       xunit.core (2.1.0)
-    xunit.abstractions (2.0.0) - framework: dnx451, dnxcore50, monoandroid, monotouch, xamarinios, winv4.5, wpv8.0, >= net45
+    xunit.abstractions (2.0.0) - framework: dnxcore50
     xunit.assert (2.1.0)
       System.Collections (>= 4.0.0) - framework: dnxcore50
       System.Diagnostics.Debug (>= 4.0.0) - framework: dnxcore50

I've determined that the issue was introduced in 2.19.0. The issue was not present in 2.18.0. Likely PR (unconfirmed) was #1174.