Break the fs package up into smaller parts.

The purpose of this is to make it easier to maintain and eventually to
allow the rclone backends to be re-used in other projects without
having to use the rclone configuration system.

The new code layout is documented in CONTRIBUTING.

fs/fspath/path_test.go

@@ -0,0 +1,35 @@
+package fspath
+
+import (
+	"fmt"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestRemoteSplit(t *testing.T) {
+
+	for _, test := range []struct {
+		remote, wantParent, wantLeaf string
+	}{
+		{"", "", ""},
+		{"remote:", "remote:", ""},
+		{"remote:potato", "remote:", "potato"},
+		{"remote:/", "remote:/", ""},
+		{"remote:/potato", "remote:/", "potato"},
+		{"remote:/potato/potato", "remote:/potato/", "potato"},
+		{"remote:potato/sausage", "remote:potato/", "sausage"},
+		{"/", "/", ""},
+		{"/root", "/", "root"},
+		{"/a/b", "/a/", "b"},
+		{"root", "", "root"},
+		{"a/b", "a/", "b"},
+		{"root/", "root/", ""},
+		{"a/b/", "a/b/", ""},
+	} {
+		gotParent, gotLeaf := RemoteSplit(test.remote)
+		assert.Equal(t, test.wantParent, gotParent, test.remote)
+		assert.Equal(t, test.wantLeaf, gotLeaf, test.remote)
+		assert.Equal(t, test.remote, gotParent+gotLeaf, fmt.Sprintf("%s: %q + %q != %q", test.remote, gotParent, gotLeaf, test.remote))
+	}
+}