Fix and refactor generator for p2p ref and tools

[jviau]

Issue describing the changes in this PR

resolves #10651

Pull request checklist

IMPORTANT: Currently, changes must be backported to the in-proc branch to be included in Core Tools and non-Flex deployments.

• Backporting to the in-proc branch is not required
  • Otherwise: Link to backporting PR
• My changes do not require documentation changes
  • Otherwise: Documentation issue linked to PR
• My changes should not be added to the release notes for the next release
  • Otherwise: I've added my notes to release_notes.md
• My changes do not need to be backported to a previous version
  • Otherwise: Backport tracked by issue/PR #issue_or_pr
• My changes do not require diagnostic events changes
  • Otherwise: I have added/updated all related diagnostic events and their documentation (Documentation issue linked to PR)
• I have added all required tests (Unit tests, E2E tests)

Additional information

Ever since moving to common artifact outputs, ExtensionsMetadataGenerator has been incorrectly packing itself due to hardcoding the output path format. This PR addresses that and performs further refactoring.

1. Assembly inclusion in the package is now done by calling into an inner-build target from the outer-build to collect all files for that TFM
2. ExtensionsMetadataGenerator.Console is now referenced via a ProjectReference, correctly establishing restore & build ordering.

[jviau]

/azp run host.public

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Azure Pipelines successfully started running 1 pipeline(s).

[jviau]

/azp run host.integration-tests

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).