fix: exception happening on startup of applications

[glennawatson]

What kind of change does this PR introduce?

bug fix.

What is the current behavior?

when loading on different platforms it will attempt to load DLLs for all the available NuGet packages we support. This will throw a FileNotFoundException.

What is the new behavior?

Does a file check to make sure the DLL is available before attempting to load the DLL.

What might this PR break?

Hopefully nobody.

Please check if the PR fulfills these requirements

• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

Other information:

[codecov]

Codecov Report

Merging #2253 into master will decrease coverage by 0.04%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #2253      +/-   ##
==========================================
- Coverage   55.86%   55.81%   -0.05%     
==========================================
  Files         114      114              
  Lines        4495     4490       -5     
  Branches      688      688              
==========================================
- Hits         2511     2506       -5     
  Misses       1803     1803              
  Partials      181      181

Impacted Files	Coverage Δ
src/ReactiveUI/Mixins/DependencyResolverMixins.cs	91.3% <100%> (+1.06%)	⬆️
src/ReactiveUI.Winforms/ReactiveUserControl.cs	50% <0%> (-11.54%)	⬇️
src/ReactiveUI.Winforms/ViewModelViewHost.cs	91% <0%> (-0.27%)	⬇️
src/ReactiveUI.Winforms/RoutedViewHost.cs	90.41% <0%> (-0.26%)	⬇️
.../ReactiveUI.Winforms/ViewModelViewHost.Designer.cs	100% <0%> (ø)	⬆️
...eactiveUI.Winforms/ReactiveUserControl.Designer.cs	100% <0%> (ø)	⬆️

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