Add command line options to manipulate imports

msbuild implicitly import files during various stages. there is no (obvious) way to inspect or modify what is imported, at the moment.

for example (https://github.com/dotnet/sdk/issues/887), sometimes we find overriding a property with /p flag is too late, but the only chance of early import relies on heuristics in finding `Directory.Build.props` -- a wildcard that might be "too greedy" (https://github.com/Microsoft/msbuild/issues/762).

can we get command line options to manipulate such behaviors? e.g. if specified, overrides the above heuristic and use a given file instead, or list the files imported, etc.

Also see https://github.com/Microsoft/msbuild/issues/3166 and https://github.com/dotnet/roslyn/blob/master/src/Workspaces/CoreTestUtilities/Resources/Directory.Build.props