do not fail in CI when package source is not provided for conf packages

[zapashcanon]

Hi,

For conf-* packages, the CI is always giving the following error:

Error in conf-X: No package source directory provided.

It would be great to remove this error for conf-* packages (those having the conf flag in the opam file) in order to unclutter the output.

[mseri]

There should already be an issue for that, right @shonfeder ?

[zapashcanon]

(I searched quickly for existing issues but did not find anything)