Move the command to cmd/sfdupes and the code to internal/sfdupes (closes #157)

`go install sneak.berlin/go/sfdupes/cmd/sfdupes@...` found no package: the command was built from `package main` in the repository root. It now lives in `cmd/sfdupes/main.go`, whose one call is `sfdupes.Main()`, and every other Go file moved with `git mv` to `internal/sfdupes`, as `REPO_POLICIES.md` and the Go style guide require. The `Makefile` builds `./cmd/sfdupes` and sets `Version` by its new import path. README "Install" and `TODO.md` give the new paths; the fleet-written "flat layout" divergence is gone.

Rule suppressed: each test file carries `//nolint:testpackage`, since the tests call unexported functions throughout.

Model: opus-5-5

internal/sfdupes/exclude_darwin.go

@@ -0,0 +1,52 @@
+//go:build darwin
+
+package sfdupes
+
+import (
+	"slices"
+
+	"golang.org/x/sys/unix"
+)
+
+// nonPersistentFilesystems returns the device ID of every mounted
+// filesystem that is not persistent, with its type, from the system's
+// list of mounted filesystems. The device ID is read with lstat on the
+// mount point. Where one filesystem is mounted over another on the same
+// mount point, the later one in the list counts. A mount point that
+// lstat cannot reach is left out.
+func nonPersistentFilesystems() (map[uint64]string, error) {
+	// The first call counts the filesystems, the second lists them.
+	n, err := unix.Getfsstat(nil, unix.MNT_NOWAIT)
+	if err != nil {
+		return nil, err
+	}
+
+	list := make([]unix.Statfs_t, n)
+
+	n, err = unix.Getfsstat(list, unix.MNT_NOWAIT)
+	if err != nil {
+		return nil, err
+	}
+
+	mounts := make(map[string]string, n)
+
+	for _, st := range list[:n] {
+		mounts[unix.ByteSliceToString(st.Mntonname[:])] =
+			unix.ByteSliceToString(st.Fstypename[:])
+	}
+
+	types := nonPersistentFilesystemTypes()
+	devices := make(map[uint64]string)
+
+	for mountPoint, fsType := range mounts {
+		if !slices.Contains(types, fsType) {
+			continue
+		}
+
+		if dev, ok := deviceOfPath(mountPoint); ok {
+			devices[dev] = fsType
+		}
+	}
+
+	return devices, nil
+}