fs: Add interface and FS implementations

This adds two implementations of the new `FS` interface: One for the local
file system (`Local`) and one for a single file read from an
`io.Reader` (`Reader`).

internal/fs/interface.go

@@ -0,0 +1,38 @@
+package fs
+
+import (
+	"io"
+	"os"
+)
+
+// FS bundles all methods needed for a file system.
+type FS interface {
+	Open(name string) (File, error)
+	OpenFile(name string, flag int, perm os.FileMode) (File, error)
+	Stat(name string) (os.FileInfo, error)
+	Lstat(name string) (os.FileInfo, error)
+
+	Join(elem ...string) string
+	Separator() string
+	Abs(path string) (string, error)
+	Clean(path string) string
+	VolumeName(path string) string
+	IsAbs(path string) bool
+
+	Dir(path string) string
+	Base(path string) string
+}
+
+// File is an open file on a file system.
+type File interface {
+	io.Reader
+	io.Writer
+	io.Closer
+
+	Fd() uintptr
+	Readdirnames(n int) ([]string, error)
+	Readdir(int) ([]os.FileInfo, error)
+	Seek(int64, int) (int64, error)
+	Stat() (os.FileInfo, error)
+	Name() string
+}