[generator] Don't process duplicate reference assemblies.

[jpobst]

When a binding project references another binding project, the list of references assemblies can contain duplicates, particularly mscorlib:

    [0]: "C:\\Program Files (x86)\\Microsoft Visual Studio\\2019\\Preview\\Common7\\IDE\\ReferenceAssemblies\\Microsoft\\Framework\\MonoAndroid\\v1.0\\mscorlib.dll"
    [1]: "C:\\code\\XamarinComponents\\Android\\BetterPickers\\source\\AndroidSwitchBackport\\bin\\Debug\\AndroidSwitchBackport.dll"
    [2]: "C:\\Program Files (x86)\\Microsoft Visual Studio\\2019\\Preview\\Common7\\IDE\\ReferenceAssemblies\\Microsoft\\Framework\\MonoAndroid\\v1.0\\Java.Interop.dll"
    [3]: "C:\\Program Files (x86)\\Microsoft Visual Studio\\2019\\Preview\\Common7\\IDE\\ReferenceAssemblies\\Microsoft\\Framework\\MonoAndroid\\v9.0\\Mono.Android.dll"
    [4]: "C:\\Program Files (x86)\\Microsoft Visual Studio\\2019\\Preview\\Common7\\IDE\\ReferenceAssemblies\\Microsoft\\Framework\\MonoAndroid\\v1.0\\mscorlib.dll"
    [5]: "C:\\Program Files (x86)\\Microsoft Visual Studio\\2019\\Preview\\Common7\\IDE\\ReferenceAssemblies\\Microsoft\\Framework\\MonoAndroid\\v1.0\\System.Core.dll"
    [6]: "C:\\Program Files (x86)\\Microsoft Visual Studio\\2019\\Preview\\Common7\\IDE\\ReferenceAssemblies\\Microsoft\\Framework\\MonoAndroid\\v1.0\\System.dll"
    [7]: "C:\\Program Files (x86)\\Microsoft Visual Studio\\2019\\Preview\\Common7\\IDE\\ReferenceAssemblies\\Microsoft\\Framework\\MonoAndroid\\v1.0\\Facades\\System.Runtime.dll"
    [8]: "C:\\Program Files (x86)\\Microsoft Visual Studio\\2019\\Preview\\Common7\\IDE\\ReferenceAssemblies\\Microsoft\\Framework\\MonoAndroid\\v1.0\\System.Xml.dll"

There is no reason to scan any duplicate assemblies a second time, so add a Distinct() to prevent duplicates.

[brendanzagaeski]

Draft release notes

Here is the note I'm planning to include for this pull request:

#### Bindings projects

- [Java.Interop GitHub PR 611](https://github.com/xamarin/java.interop/pull/611):
  Building a binding project that had a reference to another binding project
  would do a bit of redundant work for assemblies referenced by both projects.

To suggest something different or to recommend excluding it from the release notes, feel free to reply in this PR with the new suggestion. Thanks!