[MSHADE-323] make shade plugin compatible with the flatten plugin

[gzm55]

The runtime injected pom will be set via MavenProject::setFile orMavenProject::setPomFile, including this plugin and the flatten plugin. So to chain these kind of plugins, when generating the d-r-p pom, this pr first tries to parse a model directly from MavenProject::getFile and fallback to a clone of MavenProject::getOriginalModel if no pom file set.

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[jira-importer]

Resolve #323

[jira-importer]

Resolve #323