Bug in relative paths in [sources]

[KristofferC]

IIUC, currently, all relative paths for packages (pkg.path) are considered relative to the manifest file. But in cases they are provided by a [sources] entry they should in fact be treated relative to the project file. This only matters if the project and manifest file are in different directories. That situation can occur when using the manifest = entry in the project file or when using the subproject functionality in #3841.

This is a bit annoying because I think it means we need to remember from where the path entry was retrieved.