Refactor `DirectionalMotionSpecBuilder.kt` into reusable targets

- Moving builder code into a new `builder` sub package
- Extracted `DirectionalMotionSpecBuilder` in a DirectionalBuilderScope
- Extracted the implementation into a internal `DirectionalBuilderImpl`

This is done in preparation for reusing the DirectionalBuilderScope in
the new `MotionSpecBuilder`.

Test: Existing unit tests
Flag: EXEMPT refactor
Bug: 401500734
Change-Id: I5d23f341a182d65e193a064f15f9a78b10248998
6 files changed