Repro:

1. Open attached InputSystemSupportedDevices project.
2. Open Edit >> Project Settings >> Input System Package settings.
3. Verify that "Supported Devices" is a list comprising "Keyboard"
4. Open Assets/SampleInputActions.inputactioins
5. Select the <No Binding> binding of New Map >> New Action.
6. Click the "Path" dropdown

Expected result: given the action type is "Button", all the keyboard keys (which are buttons) should show up in the picker.
Actual result: Observe that only "Usages" and no keys are listed.

Verified in: 2020.1b1