Clean up project and solution file detection
Clean up project and solution file detection

Simplify search for which files might be solution files, project files, or solution filter files. Also should be *slightly* more performant, but that isn't a serious consideration here.

rainersigwald/msbuild,sean-gilliam/msbuild,rainersigwald/msbuild,cdmihai/msbuild,AndyGerlicher/msbuild,cdmihai/msbuild,AndyGerlicher/msbuild,sean-gilliam/msbuild,rainersigwald/msbuild,mono/msbuild,AndyGerlicher/msbuild,sean-gilliam/msbuild,AndyGerlicher/msbuild,mono/msbuild,rainersigwald/msbuild,sean-gilliam/msbuild,mono/msbuild,cdmihai/msbuild,mono/msbuild,cdmihai/msbuild