[MBUILDCACHE-95] Improve efficiency of cache when adding/removing modules

**[Marquis Wang](https://issues.apache.org/jira/secure/ViewProfile.jspa?name=mwang5)** opened **[MBUILDCACHE-95](https://issues.apache.org/jira/browse/MBUILDCACHE-95?redirect=false)** and commented

Right now, if you add or remove a module from your multi-module project, it invalidates your cache for your entire project, since the base pom has changed. (My project adds all modules to the dependency management of the base pom.)

This is silly, since
* When you remove a module, you should be able to reuse all of the cache
* When you add a module, you should be able to build only that one new module

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No further details from [MBUILDCACHE-95](https://issues.apache.org/jira/browse/MBUILDCACHE-95?redirect=false)

Start by tracing how the cache handles a multi-module project when its base pom and dependency management change. Compare the add-module and remove-module cases described in the issue. Done means removed modules reuse the existing cache and adding a module builds only that new module, without invalidating the whole project.

Written by the indexing model from the issue text.