Package description: files are not symbolic paths, but are globs

If you see the `PackageDescription` type then:

```
dataFiles :: [RelativePath DataDir File]
-- ^ data file globs, relative to data directory
, dataDir :: SymbolicPath Pkg (Dir DataDir)
-- ^ data directory (may be absolute, or relative to package)
, extraSrcFiles :: [RelativePath Pkg File]
, extraTmpFiles :: [RelativePath Pkg File]
, extraDocFiles :: [RelativePath Pkg File]
, extraFiles :: [RelativePath Pkg File]
```

However, these paths need to be interpreted via the glob engine before they are turned into FilePaths. It would be good to distinguish this in the types, since filepaths which contain globs are not valid paths (on windows).