check version in ci - fix precompilation

[t-bltg]

• fix alternative backend precompilation statements.;
• reduce number of using;
• move dependencies to package extensions (e.g. PGFPlotsX).

[codecov]

Codecov Report

Attention: Patch coverage is 95.50562% with 4 lines in your changes missing coverage. Please review.

Project coverage is 73.94%. Comparing base (b4e4b0d) to head (0bbbef0).
Report is 1 commits behind head on v2.

Files with missing lines	Patch %	Lines
PlotsBase/src/animation.jl	66.66%	1 Missing ⚠️
PlotsBase/src/backends.jl	87.50%	1 Missing ⚠️
PlotsBase/src/examples.jl	0.00%	1 Missing ⚠️
PlotsBase/src/init.jl	98.03%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##               v2    #5024      +/-   ##
==========================================
- Coverage   78.38%   73.94%   -4.45%     
==========================================
  Files          78       78              
  Lines        8207     8255      +48     
==========================================
- Hits         6433     6104     -329     
- Misses       1774     2151     +377     

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