[MNT] add minimal dependency management utilities

[fkiraly]

Adds minimal utilities for dependency management, to determine the packages installed.

This will be used later in isolation of dependencies that could be soft dependencies.

Instead of dumping the new utils in the current utils file, a new folder utils is added, in which the current utils is moved one level lower, and a _dependencies submodule is also added.

[codecov-commenter]

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Codecov Report

Attention: Patch coverage is 16.66667% with 10 lines in your changes missing coverage. Please review.

Project coverage is 90.01%. Comparing base (4df80af) to head (bb8f6a6).
Report is 3 commits behind head on master.

Files with missing lines	Patch %	Lines
pytorch_forecasting/utils/_dependencies.py	0.00%	10 Missing ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1628      +/-   ##
==========================================
- Coverage   90.19%   90.01%   -0.19%     
==========================================
  Files          30       32       +2     
  Lines        4724     4736      +12     
==========================================
+ Hits         4261     4263       +2     
- Misses        463      473      +10     

Flag	Coverage Δ
cpu	90.01% <16.66%> (-0.19%)	⬇️
pytest	90.01% <16.66%> (-0.19%)	⬇️

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