support using from uninstalled tree

* env.in: New file.  When configured, sets up an environment for
  Guile to load modules from an uninstalled tree.

* modules/: Rename to gcrypt/ to support loading with ./env guile.

* Makefile.am: Merge in modules/Makefile.am and update for changed
  paths.  Add EXTRA_DIST for env.in.
* modules/Makefile.am: Delete.

* configure.ac: Update.

* TODO: Remove task for renaming modules/.  Add newline at end of
  file.