When using CPS project system, fixes transform on save and displays relevant item metadata
in VS property window.

Output items for templates and preprocessed templates now have InputFIle and Dependencies
metadata.

Apparently when we tried to pack a TFM-specific file called _._ into
buildTransitive to indicate that there were no targets for that TFM,
NuGet arbitrarily decided to pack the targets file from a different TFM
instead ¯\_(ツ)_/¯

Borrow the logic from https://github.com/dotnet/runtime/blob/bb74bb79db53eae474977a0928756a0db7fc6a3f/src/libraries/Microsoft.Extensions.Configuration.Binder/src/buildTransitive/Microsoft.Extensions.Configuration.Binder.targets#L17
as it's easier than overriding NuGet's weird pack behavior.

Fixes #174

When relative pragmas are enabled, and includes already have relative
path, and there is no base bath from which to calculate relative paths,
use the existing relative path as-is.