data-files with --prefix is inconsistent between Windows and Linux

I tried reading #462 and #459 but I could not decipher what was going on in the bug discussion. So here is a report of my own. It appears that the behavior of `Paths` `getDataFile` when `--prefix` is passed is inconsistent between Linux and Windows, in the following way:

Suppose I build but do not install an executable `dist/build/myexe/myexe`, configured using `--prefix=/usr` to be installed into `usr`, which attempts to retrieve a data file. When I run this executable on Linux, it will attempt to retrieve the data file from the directory `/usr/share/ARCH-linux-ghc-VERSION/packagename-VERSION`. However, on Windows, it will attempt to retrieve the data file from the directory `dist/build/ARCH-windows-ghc-VERSION\packagename-VERSION`.

The Windows behavior is especially puzzling, because I don't believe we ever install libraries at the location we're probing. Looking at the source code, it appears that in the Windows case, `--prefix` is still triggering some sort of "relocatable" behavior (not `--enable-relocatable`, which is something different, but the codepath that occurs when `reloc` and `absolute` are false) which is causing it to be doing something different. It is very puzzling.