[xbuild] Add support for wildcards in Import.

[xbuild] Add support for wildcards in Import.

With 4.0 profile, <Import ..> can have wildcards. This works correctly
with our multiple MSBuildExtensionsPath also.

If the import references $(MSBuildExtensionsPath) property, then the
each extension path is used to resolve the import, and the search
stop as soon as an import is found.

If such an import uses wildcard, then the search stops at the
extension path in which *atleast one* file got imported.
And the import fails if any invalid project file is encountered.

This commit also heavily refactors the Import code to simplify
in the face of the new features, and improves error detection
and reporting.

ImportTest.cs: Add test for wildcards.