curate format-dates : --expected-date-formats builtins cannot be overridden

[victorlin]

continuing a side discussion from #1654

Problem

--expected-date-formats has builtin date formats that cannot be overridden. If you have dates in %Y-%d-%m form, trying by un-overrideable builtin to parse them as %Y-%m-%d is going to be a bad time.¹

¹ #1709 (comment)

Possible solutions

1. ⛔️ Change behavior so that --expected-date-formats overrides the builtins (should be termed "defaults" in this case) by using action=ExtendOverwriteDefault. This is likely to break lots of existing usage.
2. 🚧 Continue letting --expected-date-formats append to the builtins and add another way to say you don't want the builtins (e.g. --no-builtin-date-formats flag or --override-expected-date-formats=FMT … option with args or something along those lines).
   • d9f2c0d started incorporating use of the "builtin" terminology

[victorlin]

In in #1709 (comment), @joverlee521 clarified that the defaults were added after adf1e78, so I've reworded the issue description. Directly relevant discussion is also happening in that thread.

[victorlin]

#1709 addresses the first part of option 2:

2. Continue letting --expected-date-formats append to the defaults (i.e. builtins) …

As for the second part,

… and add another way to say you don't want the defaults (e.g. --no-builtin-date-formats flag or --override-expected-date-formats=FMT … option with args or something along those lines).

my vote is for --no-builtin-date-formats since I think having both --expected-date-formats=FMT … and --override-expected-date-formats=FMT … is slightly more confusing and requires more implementation logic.