For libraries the headers of private dependencies were wrongly
skipped from staging. For binaries, no headers were passed at all
in the provides map. To fix these issues, an additional field is
added in the provides map to ensure we collect, and then properly
stage, all needed headers for both libraries and binaries.
This is useful when we want to install targets built in debug mode,
but do not want to stage all the additional source and header files
if no debugging is being performed, e.g., in tests.
