Reproduction steps:

1. Open "1137372" project
2. Reimport "Foo.cs" from the Packages folder
3. Notice error "Invalid AssetDatabase path ... Use path relative to the project folder."

Expected Result: "csc.rsp" files do not generate warning messages if placed in the Package folder
Actual Result: "csc.rsp" files generate warning messages if placed in the Package folder

Fixed in: 2019.3.0a1

Reproduced with: 2019.2.0a9, 2019.1.0b8, 2019.1.0b1, 2019.1.0a15, 2019.1.0a13, 2018.3.10f1, 2018.3.0f1, 2018.3.0b11
Did not reproduce on: 2019.1.0a12, 2019.1.0a8, 2019.1.0a5, 2019.1.0a1, 2018.3.0b10, 2018.3.0b5, 2018.3.0a1, 2017.4.24f1 (No packman)