speed up import time

[oliver-sanders]

Discovered a number of cases where modules were being imported just for type checking, slowing the code needlessly. Added a flake-8 check which will flag these for future reference. Also targeted some of the biggest offenders in the critical cylc-message import pathway bringing import time down to <0.4s on my system.

• Don't import modules for type checking purposes.
• Move some light weight functions out of a heavy weight module to reduce the import penalty.
• Make a couple of heavy weight imports dynamic to avoid importing them when not necessary.
• This reduces the import time of cylc.flow.scripts.message by ~66%.
• This reduces the task runtime (script=true) by ~1s.

FYI, to investigate import time:

$ echo 'import cylc.flow.scripts.message' > testscript.py
$ python -X importtime testscript.py 2> import.log
$ pip install tuna
$ tuna import.log

Check List

• I have read CONTRIBUTING.md and added my name as a Code Contributor.
• Contains logically grouped changes (else tidy your branch by rebase).
• Does not contain off-topic changes (use other PRs for other changes).
• Applied any dependency changes to both setup.cfg (and conda-environment.yml if present).
• Tests are included (or explain why tests are not needed).
• CHANGES.md entry included if this is a change that can affect users
• Cylc-Doc pull request opened if required at cylc/cylc-doc/pull/XXXX.
• If this is a bug fix, PR should be raised against the relevant ?.?.x branch.