In-collection library lookup only works for file with `.rkt` extension

[rvs314]

I'm trying to write an R6RS library that is compatible with Racket's R6RS mode, but I'm having an issue getting the path resolution behavior working. Suppose I have the following directory structure, where the absolute path to the egg-sample directory is in current-library-collection-paths:

egg-sample/
├── lib1.scm
└── lib2.scm

If I attempt to (import (rnrs) (egg-sample lib2)) from within lib1, racket complains:

lib1.scm:5:17: import: cannot find suitable installed library
  at: (egg-sample lib2)
  in: (import (rnrs) (egg-sample lib2))
  location...:
   lib1.scm:5:17
  context...:
   /usr/share/racket/pkgs/r6rs-lib/r6rs/private/parse-ref.rkt:95:0: convert-library-reference
   /usr/share/racket/pkgs/r6rs-lib/r6rs/private/parse-ref.rkt:142:0: parse-import
   .../private/map.rkt:40:19: loop
   [repeats 1 more time]
   /usr/share/racket/pkgs/r6rs-lib/r6rs/main.rkt:179:0

However, if I rename lib2.scm to lib2.rkt, then it works correctly. The issue seems to be this invocation:

r6rs/r6rs-lib/r6rs/private/parse-ref.rkt

Lines 70 to 73 in e845408

	;; First, try specific file in collection, which works
	;; when the relevant collection is spliced:
	(path-replace-suffix
	(apply collection-file-path (path-add-suffix file #".rkt") coll)

This checks for .rkt files, but given that this is an implementation of R6RS, shouldn't it also look for .scm/.ss/.sls files as well? Is there a better way to be going about this in general? I'd be happy to send a PR if this is indeed a bug.