Linker into runtime diff2 (#78049)

Linker into runtime diff2 (#78049)

* Merge and remove common files
Remove arcade eng\common directory in src\tools\illink since now we will use the runtime arcade infra
Remove build.cmd/build.sh and lint.cmd/lint.sh in src\tools\illink directory since now they will execute via a subset
Remove/Merge common files from src\tools\illink root:
 - .gitattributes
 - .gitignore
 - .github
 - .gitmodules
 - after.illink.sln.targets
 - code_of_conduct.md
 - global.json
 - LICENSE.txt
 - NuGet.config
 - THIRD-PARTY-NOTICES.TXT
Remove/Merge common files from src\tools\illink\eng:
 - Build.props
 - Publishing.props
 - Signing.props
 - SourceBuild.props
 - SourceBuildPrebuiltBaseline.xml
 - Tools.props
 - Version.Details.xml
 - Versions.props

* Create subsets to be able to build illink
Create a variable for the tools folder in runtime
Add subsets tools.illink and tools.illinktests for building illink and unitest it
Add Microsoft.DotNet.Cecil dependencies to runtime and to illink projects
Some workarounds to be able to build illink
Delete some cecil information from the external folder since now its a package

* Refactorings to make test work
Test projects use to have relative paths based on the current working directory to know where to find stuff, now that the project is in a different place things are not found, this commit changes to instead use MSBuild variables to calculate where things are
Add the cecil package to tests
Change a cecil test that verify the official package name to only care about the important pieces

* Enable pipeline
Add a variable to recognize when illink contains a change, and set an exclusion of the src/tools/* for other repos
Reuse the dotnet-linker-tests pipeline file to also run illink unitests every time there are illink changes

* Fix Markdown lint

* Remove checked-in binaries

* Use nunit for linker tests and fix cecil version test