Add arcade-powered source-build basic configuration

[dagood]

This adds the basic arcade-powered source-build configuration. I ran this with ./build.sh -c Release --restore --build --pack /p:ArcadeBuildFromSource=true.

No patches necessary. There was only one in dotnet/source-build, a legacy feed removal that already happened here. 👍

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(Edit: adding some general text written later that might help put the PR in context a little better:)

This PR adds the local build infrastructure that lets ArPow (arcade-powered source-build) run in this repo. See https://github.com/dotnet/source-build/blob/master/Documentation/planning/arcade-powered-source-build/onboarding/local-onboarding.md for more details about how it works.

To try it out locally, run this on Linux: ./build.sh -c Release --restore --build --pack /p:ArcadeBuildFromSource=true -bl

This PR should have no effect on ordinary builds, or CI. ArPow stage 2 will add source-build to CI: PR validation and official builds.

For https://github.com/dotnet/source-build/blob/master/Documentation/planning/arcade-powered-source-build/implementation-plan.md

[dagood]

@alexperovich @ChadNedzlek will you be able to take a look at this soon? @markwilkie is listed as the owner of the repo (as far as arcade-powered source-build implementation goes) and added you to help review. Thanks.

[alexperovich]

The changes are valid msbuild/xml. After that I have no idea what they are supposed to do and if they actually do it. If it builds I guess its fine.

[ChadNedzlek]

Same as Alex. I have no idea what tweaking those random MBBuild properties is going to do. :-)

[dagood]

I'm writing up some more impl-specific docs for the next implementation stage (adding source-build to CI) and I'm going to include some notes on these files. I've been assuming people looking at this would have context on arcade-powered source-build, so I sent this out before that's ready. But, there's no reason to rush on this particular PR.

In theory you could look at https://github.com/dotnet/arcade/tree/master/src/Microsoft.DotNet.Arcade.Sdk/tools/SourceBuild to figure out what's going on, but the doc will be much more brief and specific to the files being touched here.

/cc @mmitche

[dagood]

I've added a doc about these changes to https://github.com/dotnet/source-build/blob/master/Documentation/planning/arcade-powered-source-build/onboarding/local-onboarding.md, can you take a look and see if this makes sense?

The next step of adding CI is also in there: https://github.com/dotnet/source-build/blob/master/Documentation/planning/arcade-powered-source-build/onboarding. The current plan is to ask the source-build repo champ to do that. Looks like it should be the trivial case in this repo though, since it uses the Arcade jobs template. 🙂

[dagood]

Closing in favor of #416. This PR is missing a patch that went into dotnet/source-build after I filed this PR initially.