local packages with `install-includes` fail to expose headers

How to repro:

```
$ git clone https://github.com/hvr/cabal-bugs/ && cd cabal-bugs/issue-4517

$ cabal new-build all
Resolving dependencies...
In order, the following will be built (use -v for more details):
- base-feature-macros-0.1 (lib) (first run)
- dummy-0 (exe:dummy) (first run)
Configuring library for base-feature-macros-0.1..
Preprocessing library for base-feature-macros-0.1..
Building library for base-feature-macros-0.1..
Configuring executable 'dummy' for dummy-0..
Preprocessing executable 'dummy' for dummy-0..
Building executable 'dummy' for dummy-0..

Main.hs:3:0: error:
fatal error: base-feature-macros.h: No such file or directory
#include
^
compilation terminated.
`gcc' failed in phase `C pre-processor'. (Exit code: 1)
```

This is related to a similar issue (#4120) with package-data not being exposed properly by local packages, because the package-db points to an installation folder that doesn't exist (yet).