Remove src/installer (core-setup) umbrella in favor of distinct components

[ViktorHofer]

Today, src/installer consists of various assets which are planned to either be removed or moved out of the installer directory:

• Host including end-to-end tests: Move host components out of installer into a common location #49233
• Snaps: Move dotnet-sdk snaps into the installer repo and dotnet-runtime snaps into servicing branches #49374
• Shared Framework packages: Move sfx packages out of installer into src/packs #49426
• Publishing: Use the Arcade SDK's publish.proj in dotnet/runtime arcade#8889

This issue tracks the completion of the sub-tasks and the final removal of the src/installer directory and its custom infrastructure.

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Issue Details

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Today, src/installer consists of various assets which are planned to either be removed or moved out of the installer directory:

• Host including end-to-end tests: Move host components out of installer into a common location #49233
• Snaps: Move dotnet-sdk snaps into the installer repo and dotnet-runtime snaps into servicing branches #49374
• Shared Framework packages: Move sfx packages out of installer into src/packs #49426
• Publishing: https://github.com/dotnet/core-eng/issues/12289

This issue tracks the completion of the sub-tasks and the final removal of the src/installer directory and its custom infrastructure.

Author:	ViktorHofer
Assignees:	-
Labels:	area-Infrastructure
Milestone:	6.0.0