To reproduce, create this source tree:

  libsrc\
     package.d
  main.d


The contents of libsrc\package.d:

  module lib;


The contents of main.d:

  module main;
  import lib;

  void main() {}


Map 'lib' package to 'libsrc' folder and compile:

  dmd -i -mv=lib=libsrc main.d

main.d(3): Error: module `lib` is in file 'lib.d' which cannot be read