[MBUILDCACHE-30] Track reactor projects using "implementation + dependencies" checksum instead of full

**[Alexander Ashitkin](https://issues.apache.org/jira/secure/ViewProfile.jspa?name=alex_ashitkin)** opened **[MBUILDCACHE-30](https://issues.apache.org/jira/browse/MBUILDCACHE-30?redirect=false)** and commented

Currently each project is stored within reactor using full checksum, which includes private files
Instead of that need to use "implementation + dependencies" checksum and exclude private sources which do not have dependencies

that will allow to not invalidate downstream projects if change is in private files (eg unit tests)

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This issue is a sub-task of [MBUILDCACHE-27](https://issues.apache.org/jira/browse/MBUILDCACHE-27)

Start by reading the parent MBUILDCACHE-27 and tracing how reactor projects currently calculate and store their full checksums. Done means the checksum uses implementation and dependency sources while excluding private sources without dependencies, so downstream projects are not invalidated by private-file changes such as unit-test edits.

Written by the indexing model from the issue text.