On Windows dropExeExtension doesn't actually drop the exe extension.

We have

    splitExtension "foo.exe" == ("foo", ".exe")

but the code is expecting just "exe" for the extension field.

Among other things this makes guessToolFromGhcPath behave
unexpectedly. Since takeVersionSuffix can't see past the extension the
`null suf` case in mkGuesses will trigger which causes the versioned
program path candidates to not be looked for.