[MSITE-874] Skip site:deploy instead of fail if distributionManagement.site.url is scm:*

**[Hervé Boutemy](https://issues.apache.org/jira/secure/ViewProfile.jspa?name=hboutemy)** opened **[MSITE-874](https://issues.apache.org/jira/browse/MSITE-874?redirect=false)** and commented

when deploying to SCM, [maven-scm-publish-plugin](https://maven.apache.org/plugins/maven-scm-publish-plugin/) is the way to go

instead of failing, site:deploy could simply skip deploy because it is known: this would permit simpler config for https://maven.apache.org/plugins/maven-scm-publish-plugin/examples/one-module-configuration.html 

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**Affects:** 3.9.1

Start at the site:deploy entry point and trace how distributionManagement.site.url values beginning with scm:* are handled. Compare the requested behavior with the maven-scm-publish-plugin one-module configuration example; done means site:deploy skips SCM deployment instead of failing while normal deployment remains unchanged.

Written by the indexing model from the issue text.