Reproduction steps:

1. Create a New Project
2. Import the "Post-processing" Package
3. Create a "Sprite" Object in the Hierarchy window
4. Click the Object Select Button (Circle) of the "Sprite Renderer" -> "Sprite"
5. See that there are no "LensDirtxx" Objects
6. In the Project window search for "LensDirt" (Search type "All")
7. See "LensDirtxx" Images

Expected Result: Assets in the Packages folder should be selectable in the Object Select Window
Actual Result: Assets in the Packages folder can't be used

Reproduced with: 2019.1.0a5, 2018.3.0b6, 2018.2.12f1, 2018.1.9f1, 2017.4.13f1

Note:
Although Packages only work from 2018.3