Prevent exporting terraform to the same output directory

[RemyDeWolf]

Description

Each opta layer is associated with one remote backend definition, so we don't want to allow exporting multiple opta layer into the same output directory as this would override the backend when using the --remote option.

With this change, we simplify the generate-terraform usage by expecting a distinct directory output for each layer.
If the output directory is not empty the command would fail to prevent the user to accidently alter their terraform files.

Safety checklist

• This change is backwards compatible and safe to apply by existing users
• This change will NOT lead to data loss
• This change will NOT lead to downtime who already has an env/service setup

How has this change been tested, beside unit tests?

Tested using the getting started example.

[codecov]

Codecov Report

Merging #927 (5cf7e1d) into main (f1f0644) will decrease coverage by 0.03%.
The diff coverage is 84.21%.

@@            Coverage Diff             @@
##             main     #927      +/-   ##
==========================================
- Coverage   72.83%   72.80%   -0.04%     
==========================================
  Files         120      120              
  Lines        8181     8182       +1     
==========================================
- Hits         5959     5957       -2     
- Misses       2222     2225       +3     

Flag	Coverage Δ
unittests	72.80% <84.21%> (-0.04%)	⬇️

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Impacted Files	Coverage Δ
opta/commands/generate_terraform.py	96.37% <84.21%> (-1.55%)	⬇️

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