[MBUILDCACHE-31] Implement skipping "source-only" plugin executions

**[Alexander Ashitkin](https://issues.apache.org/jira/secure/ViewProfile.jspa?name=alex_ashitkin)** opened **[MBUILDCACHE-31](https://issues.apache.org/jira/browse/MBUILDCACHE-31?redirect=false)** and commented

Need to implement following:
* add flag to cache config to mark plugin as "sourceOnly" - think of checkstyle
* if there is a change in dependencies only (eg source code checksums not changed)), skip source only plugin

Open question:
* how to restore artifacts (if any) of skipped plugins

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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 cache configuration and plugin execution handling, then review the parent MBUILDCACHE-27 task for surrounding context. Done means defining how a plugin is marked sourceOnly, skipping it when only dependencies changed, and deciding how artifacts from skipped plugins are restored.

Written by the indexing model from the issue text.