MSBuild cannot resolve simple-named dependent assemblies

### Steps to reproduce

From a task call Type.GetType() passing a simple name to an assembly which has not been loaded.

See attached project.
[msbuildRepro.zip](https://github.com/Microsoft/msbuild/files/1746006/msbuildRepro.zip)

You can run the `net46` version of the sample using desktop msbuild and see that it succeeds:
```
C:\Users\ericstj\source\msbuildRepro\task\bin\Debug\net46>msbuild /nologo /v:m
lib.MyPlugin
```

Run the `netcoreapp2.0` version of sample using dotnet msbuild and see that it fails:
```
C:\Users\ericstj\source\msbuildRepro\task\bin\Debug\netcoreapp2.0>dotnet msbuild /nologo /v:m
C:\Users\ericstj\source\msbuildRepro\task\bin\Debug\netcoreapp2.0\test.csproj(6,5): error : Could not load lib.MyPlugin,lib
```

### Expected behavior
Successfully load the simple-named assembly.

### Actual behavior
Fails to load the simple-named assembly.

The bug is here: https://github.com/Microsoft/msbuild/blob/7b4c71a46611f5f03c8d5a796ef267aa8af2ae49/src/Shared/CoreCLRAssemblyLoader.cs#L126

In this case the referenced assembly name passed in will be null (since the string didn't contain a version), but the assemblyName returned by AssemblyLoadContext will have a version (since the assembly itself has a version) so this comparison will fail. The simple fix is to change this to
```
if (assemblyName.Version != null && candidateAssemblyName.Version != assemblyName.Version)
```

I was able to test a workaround by passing the assembly version to my sample (`lib.MyPlugin,lib,Version=1.0.0.0`) which is possible here in the contrived case but won't be possible for all folks.

I think to match the CoreCLR loader behavior you might even want to candidate assembly versions that are higher than the referenced version, but that's another issue.

/cc @erozenfeld

### Environment data
`msbuild /version` output:
15.4.8.50001
`dotnet msbuild /version` output:
15.5.153.27799

If applicable, version of the tool that invokes MSBuild (Visual Studio, dotnet CLI, etc):
15.5.0-preview-007044