[xdg]: unexport xxxDir() and use xxxSubpath() only (#733)

## Summary

1. Unexport `xxxDir()`s
2. Use `xxxSubpath()` outside of pckage `xdg`
3. update `rcfilePath` based on `resolveDir`

## How was it tested?

tests

Author: hezhizhen

internal/impl/shell.go

@@ -246,7 +246,7 @@ func rcfilePath(basename string) string {
 }
 
 func fishConfig() string {
-	return filepath.Join(xdg.ConfigDir(), "fish", "config.fish")
+	return xdg.ConfigSubpath("fish/config.fish")
 }
 
 func (s *DevboxShell) Run() error {

internal/impl/shellcmd/command.go

@@ -33,7 +33,7 @@ func (c CmdFormat) String() string {
 	}
 }
 
-// ConfigShellCmds marshals and unmarshals shell commands from a devbox config
+// Commands marshals and unmarshals shell commands from a devbox config
 // as either a single string or an array of strings. It preserves the original
 // value such that:
 //

internal/xdg/xdg.go

@@ -5,38 +5,27 @@ import (
 	"path/filepath"
 )
 
-func DataDir() string {
-	return resolveDir("XDG_DATA_HOME", ".local/share")
-}
-
 func DataSubpath(subpath string) string {
-	return filepath.Join(DataDir(), subpath)
-}
-
-func ConfigDir() string {
-	return resolveDir("XDG_CONFIG_HOME", ".config")
+	return filepath.Join(dataDir(), subpath)
 }
 
 func ConfigSubpath(subpath string) string {
-	return filepath.Join(ConfigDir(), subpath)
-}
-
-func CacheDir() string {
-	return resolveDir("XDG_CACHE_HOME", ".cache")
+	return filepath.Join(configDir(), subpath)
 }
 
 func CacheSubpath(subpath string) string {
-	return filepath.Join(CacheDir(), subpath)
-}
-
-func StateDir() string {
-	return resolveDir("XDG_STATE_HOME", ".local/state")
+	return filepath.Join(cacheDir(), subpath)
 }
 
 func StateSubpath(subpath string) string {
-	return filepath.Join(StateDir(), subpath)
+	return filepath.Join(stateDir(), subpath)
 }
 
+func dataDir() string   { return resolveDir("XDG_DATA_HOME", ".local/share") }
+func configDir() string { return resolveDir("XDG_CONFIG_HOME", ".config") }
+func cacheDir() string  { return resolveDir("XDG_CACHE_HOME", ".cache") }
+func stateDir() string  { return resolveDir("XDG_STATE_HOME", ".local/state") }
+
 func resolveDir(envvar string, defaultPath string) string {
 	dir := os.Getenv(envvar)
 	if dir != "" {