[compilation database project] only sources in build folders are listed

When using the compilation database project manager, it seems that only the sources in the build folder are used to populate the targets, sources in the source folder are not listed (seen in meson and in gn projects)

not sure if there are fundamental issues preventing this, but ideally sources in the source folder would also be listed.

steps to reproduce:

• use a meson-based project (like gstreamer) or a gn based project (like chromium)
• generate the compilation database for an out-of-tree build