Fix for PkgConfig cache orphaned entries bug (xam bug #11133)

This patch:
* adds a new assembly Mono.XBuild.Tasks.dll, which contains prev. internal classes of Microsoft.Build.Tasks
* makes prev. internal classes LibraryPcFileCache and PcFileCache public for easier unit-testing
* adds a bunch of tests on class PcFileCache

This patch is MIT/X11 licensed.