`System.FilePath.Posix` treats filenames with leading `.`s as extensions

Right now `System.FilePath.Posix.splitExtension` will treat any leading `.` in a path as an extension. This leads to some odd behaviour:

```haskell
import System.FilePath.Posix

splitExtension "myrepo/.git"
-- returns ("myrepo/", ".git")

splitExtension "myrepo/.."
-- returns ("myrepo/.",".")
```

I'd expect for `splitExtension` to treat a file whose leading character is a `.` as a filename. This appears to be consistent across all functions that work on paths (`takeBaseName`, etc). Changing these would be a pretty big breaking change, but at the very least this should probably be mentioned in the docs.