First search in installed packages

[tmi]

1/ Adding a new source for searching for libs -- in installed python binary packages. Those are produced eg here https://github.com/ecmwf/eckit/pull/152/commits . This additionally extend ld library path for transitive dependencies, assuming the found libs declare them accordingly
2/ Refactored the body of the main function so that each source is separated in its own (private) function
3/ Fine-grained control over which sources are allowed for package search using environment variables

[FussyDuck]

All committers have signed the CLA.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 16.32653% with 82 lines in your changes missing coverage. Please review.

Project coverage is 35.43%. Comparing base (39345dd) to head (ab59d12).

Files with missing lines	Patch %	Lines
findlibs/__init__.py	0.00%	82 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop      #14      +/-   ##
===========================================
- Coverage    41.91%   35.43%   -6.48%     
===========================================
  Files            4        6       +2     
  Lines          136      206      +70     
===========================================
+ Hits            57       73      +16     
- Misses          79      133      +54     

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