[MSBuild] Reload projects after parsing sln file

When building sln files, they are first converted to sln.proj
files, then the project file is built.

The engine unloaded and reloaded the sln.proj file itself, but
not the projects implicitly loaded while parsing the sln file.
This meant that some state was kept around that prevented
AfterTargets from being handled correctly

BXC18761 - AfterTargets and AfterBuild ignored by xbuild