`data-files` of relocatable libraries are inaccessible in template Haskell

**Describe the bug**

The generated `Paths_...` module for `data-files` of `--enable-relocatable` packages produces invalid paths under template Haskell.

If a package, say `provides-data-file`, exposes the path to a `data-file` and is `configure`d with `--enable-relocatable`; and another package, say `embeds-data-file`, tries to access that `data-file` within template Haskell, then this access will fail with `openFile: does not exist`. The reason seems to be that for `--enable-relocatable` the generated `Paths_...` module uses paths relative to `getExecutableName` which, under template Haskell, returns the GHC executable. So, the resulting path points into the GHC installation, instead the package installation.

**To Reproduce**

```
$ git clone https://github.com/aherrmann/cabal_relocatable_data_files_template_haskell.git
$ cd cabal_relocatable_data_files_template_haskell
$ ./issue.sh
```

This should fail with the following error:

```
[1 of 1] Compiling UsesDataFile ( UsesDataFile.hs, dist/build/UsesDataFile.o )

UsesDataFile.hs:9:11: error:
• Exception when trying to run compile-time code:
/opt/ghc/8.8.2/lib/ghc-8.8.2/share/x86_64-linux-ghc-8.8.2/provides-data-file-0.1.0.0/my-data-file: openFile: does not exist (No such file or directory)
Code: do content <- runIO $ readFile =<< dataFile
[| content |]
pending(tc) []
• In the untyped splice:
$(do content <- runIO $ readFile =<< dataFile
[| content |])
|
9 | content = $(do
| ^^^^...
```

The `data-file` path is provided like this:
```
dataFile :: IO FilePath
dataFile = fmap ( "my-data-file") getDataDir
```
and consumed in template Haskell like this:
```
content :: String
content = $(do
content <- runIO $ readFile =<< dataFile
[|content|])
```

This uses `Cabal` the library by executing `runghc Setup.hs` directly.

**Expected behavior**

Compilation should succeed without error. The `data-file` path of a `--enable-relocatable` package should point to the correct location also under template Haskell.

**System information**
- Ubuntu 19.10
- GHC 8.8.2 (installed from `http://ppa.launchpad.net/hvr/ghc/ubuntu`)
- Cabal3.0.1.0

**Additional context**
This was discovered in the context of `rules_haskell`, see https://github.com/tweag/rules_haskell/issues/1246. The `shake` library tries to embed `js-jquery` using `file-embed` when configured with `-f embed-files`. In `rules_haskell` we use `--enable-relocatable` to ensure that no absolute paths leak into the build artifacts, see https://github.com/tweag/rules_haskell/issues/874#issuecomment-492147644. This is a requirement for distributed caching to work across machines and developers under Bazel.