Few fixes in CreateWixBuildWixpack task

[NikolaMilosavljevic]

Few fixes for issues uncovered while testing shared infra, consumed by runtime and windowsdesktop.

Fixes:

• ProjectDir property might not be present - some projects are not wixproj-based
• Enable resolution of included files, via custom relative path. When a source-file (WXS) includes another file, i.e. a WXI, included file is often specified as relative path. Now, we are resolving this path relatively to the path of the source-file. Originally we always used project path, but that is not always available. New model is more robust anyway.
• Fixed few bugs in ProcessReferencedIncludeFiles method
• Added support for system variables in wix conditions - EvaluateCondition method

Testing

• Tested with all runtime installers, using pending changes in WiX v5 for Microsoft.DotNet.Build.Tasks.Installers #15933 and Migrate to WiX 5 runtime#117010. This included testing of repacking of produced wixpacks.
• Regression testing: wixpack creation of all sdk and aspnetcore installers