This is a follow-up to commit 5ffac538aa604b71814ac74579626f0d3110b96e.
* tests/pack.scm (self-contained-tarball): Adjust arguments to
"self-contained-tarball".
* guix/scripts/pack.scm (<compressor>)[package]: Remove.
[command]: Document as being a gexp with an absolute file name.
(%compressors): Adjust accordingly.
(self-contained-tarball): Simplify PATH expression. Move 'string-join'
for the compressor command on the build side.
(docker-image): Simplify PATH expression.
* tests/pack.scm (%gzip-compressor): Adjust accordingly.
* guix/scripts/pack.scm (self-contained-tarball): Add #:tar option.
[build](tar-supports-sort?): New variable. Use it.
* tests/pack.scm: New file.
* Makefile.am (SCM_TESTS): Add it.
