[MSITE-807] support staging of symbolic links

**[Hervé Boutemy](https://issues.apache.org/jira/secure/ViewProfile.jspa?name=hboutemy)** opened **[MSITE-807](https://issues.apache.org/jira/browse/MSITE-807?redirect=false)** and commented

as seen during maven-scm-publish-plugin MSCMPUB-35 work, having it not blow up on symbolic links is a first step to be able to publish mono-module sites
but for multi-module sites, maven-site-plugin needs to be able to stage the multi-modules site before scm-publish can publish it

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**Affects:** 3.7

**Issue Links:**
- [MSCMPUB-35](https://issues.apache.org/jira/browse/MSCMPUB-35) add support for symbolic links

Start by reviewing MSITE-807 and the linked MSCMPUB-35 issue to understand the symbolic-link staging problem and its multi-module site context. Done means the Maven Site Plugin can stage multi-module sites containing symbolic links without blowing up, so scm-publish can publish them.

Written by the indexing model from the issue text.