Make TransitModelIndex more immutable

[habrahamsson-skanetrafiken]

Summary

In this PR:

• Hide most of the maps in TransitModelIndex.
• Make collections returned by TransitModelIndex unmodifiable.
• Remove functionality from TransitModelIndex that already exists in TransitModel.

Issue

See #6048

Unit tests

Some of the stuff in TransitModelIndex is untested. I didn't add any tests.

Documentation

Didn't add any documentation.

Bumping the serialization version id

I guess, since I'm changing the signature of the TransitModel?

[leonardehrenfried]

Needs to be even immutabler!

[codecov]

Codecov Report

Attention: Patch coverage is 62.16216% with 14 lines in your changes missing coverage. Please review.

Project coverage is 69.84%. Comparing base (9e58db4) to head (629562b).
Report is 84 commits behind head on dev-2.x.

Files with missing lines	Patch %	Lines
...planner/transit/service/DefaultTransitService.java	56.25%	7 Missing ⚠️
...tripplanner/transit/service/TransitModelIndex.java	64.70%	5 Missing and 1 partial ⚠️
...in/java/org/opentripplanner/netex/NetexModule.java	0.00%	1 Missing ⚠️

Additional details and impacted files

@@              Coverage Diff              @@
##             dev-2.x    #6140      +/-   ##
=============================================
- Coverage      69.85%   69.84%   -0.02%     
- Complexity     17491    17669     +178     
=============================================
  Files           1976     1995      +19     
  Lines          74712    75267     +555     
  Branches        7645     7702      +57     
=============================================
+ Hits           52192    52571     +379     
- Misses         19868    20024     +156     
- Partials        2652     2672      +20     

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