Exclude dependency from RequiredModules when listed under ExternalModuleDependencies

[anamnavi]

PSResourceGet currently requires all modules to be found/installed if present under RequiredModules, even if additionally present under ExternalModuleDependencies. RequiredModules is referred to for the following operations: Install-PSResource, Publish-PSResource and Import-Module.

Previously, PSResourceGet expected the user to not list an external dependency under RequiredModules but this breaks Import-Module. This PR addresses a fix for Install-PSResource. If the module is listed under ExternalModuleDependencies, it is skipped during installation and a verbose message is written. As the ExternalModuleDependencies metadata is not returned by the PSGallery server during Find call, it must be retrieved from the .psd1 or .ps1 metadata file for a PSResource.

#1809 also discusses checking the PSModulePath for an ExternalModule dependency and warning the user accordingly, but this PR does not address this yet.

PR Summary

PR Context

Resolves #1809

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[anamnavi]

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