add --in-place flag to verdi export migrate

[ltalirz]

Fixes #4233
Fixes #4322

When export migration is required, you often want to get rid of the outdated
export file.
The --in-place flag saves users from specifying a temporary filename,
removing the old file and renaming the temporary one.

This commit also changes the exit code for migrating an export file that
already is up to date from an error to success (0).

[codecov]

Codecov Report

Merging #4220 into develop will decrease coverage by 0.02%.
The diff coverage is 82.36%.

@@             Coverage Diff             @@
##           develop    #4220      +/-   ##
===========================================
- Coverage    79.34%   79.32%   -0.01%     
===========================================
  Files          468      468              
  Lines        34737    34752      +15     
===========================================
+ Hits         27558    27565       +7     
- Misses        7179     7187       +8     

Flag	Coverage Δ
#django	72.95% <82.36%> (-0.01%)	⬇️
#sqlalchemy	72.15% <82.36%> (-<0.01%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
aiida/cmdline/commands/cmd_export.py	90.45% <82.36%> (-2.94%)	⬇️
aiida/engine/daemon/client.py	72.42% <0.00%> (-1.14%)	⬇️
aiida/transports/plugins/local.py	81.29% <0.00%> (-0.25%)	⬇️

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[sphuber]

Thanks @ltalirz some small changes

[CasperWA]

Seems fine to me.

I have some suggested updates to parts of the tests, nothing serious.
What's more serious is that you are effectively implementing/changing two things in this PR, one is adding the --in-place flag option, but then you're also changing expected behaviour of the migration functionality.
The latter might be fine, but where has this change been discussed?

[ltalirz]

@sphuber This should be ready for review