No current-load-relative-directory in expander

It seems like (current-load-relative-directory) is unavailable during background expansion and no working REPL loaded, while the code runs and compiles just fine from the command line:

#lang racket

(require (for-syntax syntax/parse/pre
                     compiler/cm-accomplice))

(define-syntax (require/phantom stx)
  (syntax-parse stx
    [(_ relpath:string)
     (register-external-module (path->complete-path (syntax-e #'relpath)
                               (current-load-relative-directory)))
     #'(begin)]))

(require/phantom "test.rkt")

#|
Welcome to DrRacket, version 8.15.0.12 [cs].
Language: racket, with debugging [custom]; memory limit: 128 MB.
. . ../../racket/gitdev/racket/collects/syntax/wrap-modbeg.rkt:46:4: path->complete-path: contract violation
  expected: (and/c (or/c path-string? path-for-some-system?) complete-path?)
  given: #f
> 
|#

Here, (current-load-relative-directory) returns #f in DrRacket but not in Racket REPL:

● [Jan10 20:47:44] $ racket
Welcome to Racket v8.15.0.12 [cs].
> (enter! "phantom-require.rkt")
"phantom-require.rkt"> 

During compilation, (current-load-relative-directory) also returns the load path:

● [Jan10 20:47:54] $ raco make -v phantom-require.rkt 
"phantom-require.rkt":
  making #<path:....../phantom-require.rkt>
 [output to "compiled/phantom-require_rkt.zo"]
● [Jan10 20:48:04] $ raco make -v phantom-require.rkt
"phantom-require.rkt":
  making #<path:....../test.rkt>
test.rkt:(syntax-local-phase-level 0)
  making #<path:....../phantom-require.rkt>
 [output to "compiled/phantom-require_rkt.zo"]
● [Jan10 20:48:06] $ 

Start with the require/phantom reproducer and the current-load-relative-directory call, then compare its value during DrRacket background expansion, in the Racket REPL, and with raco make. Done means the DrRacket case no longer produces #f for this use or otherwise avoids the reported contract violation.