"You must give at least one requirement to install" on rebuild

$ snapcraft --shell-after
…
Snapped offlineimap_v7.1.1+git6.7067be9_amd64.snap

$ snapcraft --shell-after
Launching a VM.
snap "core" has no updates available
SNAPCRAFT_BUILD_ENVIRONMENT_CHANNEL_SNAPCRAFT is set: installing snapcraft from latest/stable
snap "snapcraft" has no updates available
snap "core18" has no updates available
Hit http://archive.ubuntu.com/ubuntu bionic InRelease
Hit http://archive.ubuntu.com/ubuntu bionic-updates InRelease
Hit http://archive.ubuntu.com/ubuntu bionic-backports InRelease
Hit http://security.ubuntu.com/ubuntu bionic-security InRelease
Fetched 0 B in 0s (0 B/s)
Updating pull step for offlineimap (source changed)
Updating build step for offlineimap ('pull' step changed)
DEPRECATION: Python 2.7 will reach the end of its life on January 1st, 2020. Please upgrade your Python as Python 2.7 won't be maintained after that date. A future version of pip will drop support for Python 2.7.
Processing /root/parts/offlineimap/build
  Link is a directory, ignoring download_dir
Successfully downloaded offlineimap
DEPRECATION: Python 2.7 will reach the end of its life on January 1st, 2020. Please upgrade your Python as Python 2.7 won't be maintained after that date. A future version of pip will drop support for Python 2.7.
Looking in links: /root/parts/offlineimap/python-packages
Processing /root/parts/offlineimap/build
Building wheels for collected packages: offlineimap
  Building wheel for offlineimap (setup.py) ... done
  Stored in directory: /tmp/tmp_t9tpbir
Successfully built offlineimap
DEPRECATION: Python 2.7 will reach the end of its life on January 1st, 2020. Please upgrade your Python as Python 2.7 won't be maintained after that date. A future version of pip will drop support for Python 2.7.
DEPRECATION: Python 2.7 will reach the end of its life on January 1st, 2020. Please upgrade your Python as Python 2.7 won't be maintained after that date. A future version of pip will drop support for Python 2.7.
ERROR: You must give at least one requirement to install (maybe you meant "pip install /root/parts/offlineimap/python-packages"?)
Failed to run '/root/stage/usr/bin/python2 -m pip install --user --no-compile --no-index --find-links /root/parts/offlineimap/python-packages --upgrade --no-deps': Exited with code 1.
Run the same command again with --debug to shell into the environment if you wish to introspect this failure.
                                                                                                                      An error occurred when trying to execute 'sudo -i snapcraft snap' with 'multipass': returned exit code 2.

$ cat snapcraft.yaml
name: offlineimap
version: git
summary: OfflineIMAP
description: |
  OfflineIMAP is software that downloads your email mailbox(es) as local
  Maildirs. OfflineIMAP will synchronize both sides via IMAP.

confinement: devmode
base: core18

parts:
  offlineimap:
    plugin: python
    python-version: python2
    source: .
    stage-packages:
      - python-six

apps:
  offlineimap:
    command: bin/offlineimap