Support missing ecosystem

[jasinner]

Deplist has many dependencies and users may not want to install and keep them all up to date. Also, they may not need to support Ruby, or Java projects, because they have an alternative solution such as Syft for that. In that case Deplist should gracefully handle a missing ecosystem library.

In order to avoid the situation where a library is accidentally not installed, the default operating mode should remain as it is (fail on missing ecosystem library). We should have a 'slim' operating mode where only the installed ecosystems are used for scanning.