Path.IsPathRooted vs Path.IsPathFullyQualified

[JanProvaznik]

We're using Path.IsPathRooted in a lot of places in MSBuild with the intended semantics of https://learn.microsoft.com/en-us/dotnet/api/system.io.path.ispathfullyqualified?view=net-9.0

Tracking issue for validating this interpretation is correct and migrating all usage to Path.IsPathFullyQualified

slight issue is that the method is not available for netstandard

• suggested workaround: create a util which fallbacks to ispathrooted for netstandard in conditional compilation, so it compiles successfully. we don't really run the assembly targeting netstandard.