[BUG] Cygdb does not process positional BUILD_DIR argument as expected

[avm19]

Describe the bug

Cygdb CLI commands (e.g. cygdb /path/to/debug/info, cygdb --) do not process the BUILD_DIR argument
(aka "path_to_debug_info") as expected.

A number of user guides (Cython's, Pandas', @WillAyd's) expect that when a user types cygdb, Cygdb will import debug information from the current directory, which is currently broken. This is one example, see the table below for more. I believe this should be fixed in master asap, in order to keep Cygdb friendly to new users.

I assume that intended behaviour is described in the User Guide - Debugging (~15 years old), and that the only desired change
since 3.0.x is the addition of --build-dir /path/to/debug/info, which used to be positional (now deprecated).

Here is a summary of intended vs actual (current) behaviour for various CLI calls.

command	3.0.x intended	3.0.x actual	master intended	master actual
cygdb	import curdir	✔ import curdir	import curdir	❌ no import1
cygdb .	import curdir	✔ import curdir	import curdir	❌ no import1
cygdb /path/	import /path/	❌ import curdir2	import /path/	❌ no import1
cygdb --	no import	❌ import curdir2	no import	⚠ no import3
cygdb --build-dir .	N/A	✔ error	import curdir	✔ import curdir
cygdb --build-dir /path/	N/A	✔ error	import /path/	✔ import /path/

Code to reproduce the behaviour:

# example code

Expected behaviour

No response

OS

No response

Python version

3.10.16+

Cython version

master

Additional context

I propose the following.

1. A "quick fix" PR for the master branch will be supplied in short time.
2. If needed, I can produce another "quick fix" for related issues in 3.0.x branch.
   However, the broken behaviour seems to have been around for a while, so some users may actually rely on it.
3. Discuss changes in cygdb command arguments to completely move away from positional arguments to keywords.
   In particular, cygdb -- could be deprecated by introducing --no-import key,
   as was initially proposed in Upgrade cygdb from optparse to argparse #5499 (but not included in the final version).

Footnotes

1. These bugs were introduced in Upgrade cygdb from optparse to argparse #5499. ↩ ↩² ↩³

2. These bugs were introduced in 60dff16 when moving from sys.argv to optparse.
   Unlike sys.argv, optparse and argparse do not regard -- as an argument in cygdb --. ↩ ↩²

3. The correct behaviour is achieved by accident, the code is not doing what one might expect. ↩