tailfs: clean up naming and package structure

- Restyles tailfs -> tailFS
- Defines interfaces for main TailFS types
- Moves implemenatation of TailFS into tailfsimpl package

Updates tailscale/corp#16827

Signed-off-by: Percy Wegmann <percy@tailscale.com>

tailfs/tailfsimpl/shared/pathutil.go

@@ -0,0 +1,42 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+package shared
+
+import (
+	"path"
+	"strings"
+)
+
+// This file provides utility functions for working with URL paths. These are
+// similar to functions in package path in the standard library, but differ in
+// ways that are documented on the relevant functions.
+
+const (
+	sepString       = "/"
+	sepStringAndDot = "/."
+	sep             = '/'
+)
+
+// CleanAndSplit cleans the provided path p and splits it into its constituent
+// parts. This is different from path.Split which just splits a path into prefix
+// and suffix.
+func CleanAndSplit(p string) []string {
+	return strings.Split(strings.Trim(path.Clean(p), sepStringAndDot), sepString)
+}
+
+// Join behaves like path.Join() but also includes a leading slash.
+func Join(parts ...string) string {
+	fullParts := make([]string, 0, len(parts))
+	fullParts = append(fullParts, sepString)
+	for _, part := range parts {
+		fullParts = append(fullParts, part)
+	}
+	return path.Join(fullParts...)
+}
+
+// IsRoot determines whether a given path p is the root path, defined as either
+// empty or "/".
+func IsRoot(p string) bool {
+	return p == "" || p == sepString
+}