Better logging for multi-project scans

Proposal

Some of our SAST analyzers have multi-project support. There is a log message in common that confuses the fact that an analyzer scan w/ multi-project support is actually scanning multiple projects/apps within a repo.

Relevant discussion: #281586 (comment 464299716)

Implementation

If https://gitlab.com/gitlab-org/security-products/analyzers/common/-/blob/master/command/command.go#L21 is set then the log message should be "Detected relevant project, scanning root" or something along those lines.