BREAKING CHANGE: Disable scanning OS packages in fs mode

[DmitriyLewen]

Motivation

Initially, Trivy only used fs mode.
We later split it into fs and rootfs to separate pre-build and post-build.

We currently keep the ability to scan OS packages in fs mode for backward compatibility.

But it has been a while since we added the rootfs mode.
So we decided to disable scanning OS packages in fs mode (you can still scan OS packages in rootfs and image modes).

[knqyf263]

Pre-build and post-build were defined to target scan artifacts that might be duplicated, such as pom.xml and JAR files. Following this definition, OS packages should be disabled in a filesystem scan. However, if scanning OS packages during a filesystem scan doesn't cause any issues, there is also the option to leave it enabled.

On the other hand, for new users, both fs and rootfs scanning OS packages might make it harder to distinguish the difference between them, which could be seen as a downside. Since each approach has pros and cons, we need further discussion.