…ity (sbt#8960)

**Problem**
sbt always includes all transitive dependencies on the classpath.
This makes it easy to accidentally depend on transitive dependencies
without declaring them, leading to fragile builds that break when
a library changes its own dependencies.

**Solution**
Add a `dependencyMode` setting with three modes:

- DependencyMode.Transitive (default) — current behavior, all
  transitive dependencies on the classpath
- DependencyMode.Direct — only declared dependencies plus
  scala-library on the classpath
- DependencyMode.PlusOne — declared dependencies plus their
  immediate transitive dependencies plus scala-library

Fixes sbt#8942