data-dir mismatch when installing component

**Describe the bug**
When configuring, building, and installing a package component, instead of the whole package, Cabal will generate a `datadir` value in the `Paths_*` module that does not match the actual installation `datadir`. Potentially leading to `file not found` errors at runtime of the install package component.

**To Reproduce**

Fetch and unpack the `alex` source distribution
```
$ curl -L http://hackage.haskell.org/package/alex-3.2.4/alex-3.2.4.tar.gz | tar xz
$ cd alex-3.2.4
```

Configure and install the `exe:alex` component
```
$ mkdir install_dir
$ runghc Setup.hs configure exe:alex --user --prefix=$PWD/install_dir --datadir=$PWD/install_dir/alex_data
$ runghc Setup.hs build
$ runghc Setup.hs install
```

Observe a mismatch in the actual `datadir` and the `datadir` expected by `Paths_alex`:
```
$ grep 'datadir\s*=' dist/build/alex/autogen/Paths_alex.hs
datadir = "/tmp/alex-3.2.4/install_dir/alex_data/x86_64-linux-ghc-8.6.5/alex-3.2.4-BnUEMKTG1CcF5xN1sTDkoW-alex"
$ ls install_dir/alex_data/x86_64-linux-ghc-8.6.5/
alex-3.2.4
```

**Expected behavior**
The `datadir` value in `Paths_alex.hs` should match the actual installation directory.

**System informataion**
- Operating system: 5.1.4-1-default x86_64 GNU/Linux
-
```
$ runghc Setup.hs --version
Cabal library version 2.4.0.1
```

**Additional context**
This issue can be worked around by setting the `--datasubdir=` flag, which overrides the default subdirectory under `--datadir` and does not depend on the component.

Reproduce the mismatch with the provided Setup.hs configure, build, and install commands, then compare the generated dist/build/alex/autogen/Paths_alex.hs datadir with the installed directory under install_dir/alex_data. Trace how the component and --datasubdir settings determine these paths; done means the generated datadir matches the actual installation directory for this case.

Written by the indexing model from the issue text.