De-duplicate initialize_goto_model's code [blocks: #5837]

[tautschnig]

lazy_goto_modelt used almost the same code. Refactor
initialize_goto_model's implementation to enable re-use from
lazy_goto_modelt.

• Each commit message has a non-empty body, explaining why the change was made.
• Methods or procedures I have added are documented, following the guidelines provided in CODING_STANDARD.md.
• n/a The feature or user visible behaviour I have added or modified has been documented in the User Guide in doc/cprover-manual/
• Regression or unit tests are included, or existing tests cover the modified code (in this case I have detailed which ones those are in the commit message).
• n/a My commit message includes data points confirming performance improvements (if claimed).
• My PR is restricted to a single feature or bugfix.
• n/a White-space or formatting changes outside the feature-related changed lines are in commits of their own.

[codecov]

Codecov Report

Merging #6697 (ad9c19e) into develop (05e1fd6) will increase coverage by 0.04%.
The diff coverage is 87.25%.

@@             Coverage Diff             @@
##           develop    #6697      +/-   ##
===========================================
+ Coverage    76.89%   76.93%   +0.04%     
===========================================
  Files         1582     1583       +1     
  Lines       183170   183288     +118     
===========================================
+ Hits        140840   141011     +171     
+ Misses       42330    42277      -53     

Impacted Files	Coverage Δ
jbmc/src/java_bytecode/lazy_goto_model.h	100.00% <ø> (+7.40%)	⬆️
src/ansi-c/expr2c_class.h	100.00% <ø> (ø)
src/crangler/ctokenit.cpp	0.00% <0.00%> (ø)
src/crangler/mini_c_parser.cpp	72.67% <0.00%> (ø)
src/goto-checker/solver_factory.h	100.00% <ø> (ø)
src/goto-instrument/reachability_slicer_class.h	100.00% <ø> (ø)
src/goto-programs/goto_trace.h	100.00% <ø> (ø)
src/memory-analyzer/analyze_symbol.cpp	72.98% <0.00%> (ø)
src/symtab2gb/symtab2gb_parse_options.cpp	63.38% <0.00%> (ø)
src/util/exception_utils.h	92.85% <ø> (ø)
... and 33 more

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