Do not rely on Base.active_repl

[serenity4]

Relying on Base.active_repl introduced two failure modes:

• If Cthulhu.REPL !== parentmodule(typeof(Base.active_repl)), we error.
• If we launch in non-interactive mode, Base.active_repl is not defined and we error.

We used it only to retrieve a terminal, but as it turns out the setup is extremely simple so it seems we can just do that ourselves. From what I understand, it hooks into stdout/stdin/stderr, but nothing requires interacting with Base.active_repl.

This should fix #450.

Reproducing the first failure mode (and ensuring it is now fixed) is a bit tricky, these conditions seem to work for me:

• Copy the REPL package from its stdlib location to somewhere else (e.g. ~/.julia/dev).
• dev it within the Cthulhu project.
• Launch Julia with julia --project --startup-file=no then execute using Cthulhu; @descend exp(4).
• This should error on master and pass on this PR.

The second failure mode can be reproduced with #450 (comment).

[codecov-commenter]

Codecov Report

❌ Patch coverage is 0% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 0.00%. Comparing base (2c33b9a) to head (53e6772).
⚠️ Report is 99 commits behind head on master.

Files with missing lines	Patch %	Lines
src/CthulhuBase.jl	0.00%	4 Missing ⚠️

Additional details and impacted files

@@          Coverage Diff           @@
##           master    #643   +/-   ##
======================================
  Coverage    0.00%   0.00%           
======================================
  Files           9      11    +2     
  Lines        1556    1592   +36     
======================================
- Misses       1556    1592   +36     

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[aviatesk]

Thanks, confirmed this fixes the both failure modes.