Add support for source-build

[JunTaoLuo]

Addresses #3752

[JunTaoLuo]

@aspnet-hello triage https://dev.azure.com/dnceng/public/_build/results?buildId=253104&view=logs

[aspnet-hello]

This comment was made automatically. If there is a problem contact aspnetcore-build@microsoft.com.

I've triaged the above build. I've created/commented on the following issue(s)
https://github.com/aspnet/AspNetCore-Internal/issues/2771

[dougbu]

Just a couple of outstanding points, including a few on earlier iterations

[dougbu]

Hurray:grey_exclamation:

[sharpninja]

What is the expected happy path for building the repository from scratch on Ubuntu 18.04 in WSL 2? I'm using this script:

#!/usr/bin/env bash

cd /mnt/e/GitHub/sharpninja

rm -r -f aspnetcore

git clone --recursive git@github.com:sharpninja/AspNetCore.git

git submodule update --init --recursive

cd aspnetcore

git remote add upstream https://github.com/aspnet/aspnetcore
git fetch upstream
git merge upstream/master

./restore.sh
./build.sh --all --restore -c Debug -v d

Unfortunately, it attempts to import internal.aspnetcore.buildtasks/3.0.0-build-20190530.3 twice and bombs out on the second attempt. I'm attaching a log file.
aspnet-build.log

[dougbu]

@sharpninja it sounds like you're having an issue with building this repo locally. If that's the case, commenting on a merged PR is not likely to get attention. Please file a new issue if my comments don't help.

Suggest leaving --restore out of the ./build.sh command. In addition, execute git submodule update --init --recursive much later, after updating your local 'master' branch and just before ./restore.sh.

In case your fork contains commits that are causing problems, I also suggest git reset --hard upstream/master instead of git merge upstream/master.