https://pip.pypa.io/en/stable/user_guide/#installing-from-local-packages states that you can use a --directory option with the install subcommand. This fails with pip 18.1 on both Python 2.7 on SLES12 and Python 3.6 on Windows 10:

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pip install --download my_dir -r reqs.txt

Usage:
  pip install [options] <requirement specifier> [package-index-options] ...
  pip install [options] -r <requirements file> [package-index-options] ...
  pip install [options] [-e] <vcs project url> ...
  pip install [options] [-e] <local project path> ...
  pip install [options] <archive url/path> ...

no such option: --download