Creating the MSBuild project in-memory

Now that we're no longer running the MSBuild command line, the next isolating refactoring is to create the project file in memory, too. To do that, I used ProjectRootElement:

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Note this depends on the following extension method, which I introduced to reduce duplication:

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I have an urge to write an extension method to replace AddTask which will ensure that the project has a UsingTask already. In fact, I'll go ahead and do that, so you can see the transformation.

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I also made AddUsingTask generic:

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Which allows the following simpler test:

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