Proposal: manage all the source code in one repo

# Background
The CompoundVM JDK was built from source code of two repositories: [modified jdk17u (aka this-repo)](https://github.com/bytedance/CompoundVM/), [unmodified jdk8u](https://github.com/openjdk/jdk8u.git). and the jdk8u source code was downloaded using wget at build time, see [cvm.mk:150](https://github.com/bytedance/CompoundVM/blob/jdk17u-target8/cvm.mk#L150).

This approach has two drawbacks:
- the external dependency may be unstable, affected by network, permission, etc.;
- it would be hard to maintain changes in the downloaded package;

# Proposal
This proposal tries to maintain all the source code in one repository (this one).

# Design

## multiple jdk*u upstreams
- add different upstreams using `git remote`, below commands use jdk8u as example.
```shell
# add the jdk8u remote
cd compoundVM/
git remote add jdk8u https://github.com/openjdk/jdk8u.git
git fetch jdk8u

# create a local branch to track jdk8u/master
git branch jdk8u_master -t jdk8u/master

# set up a new worktree in sub-dir cvm/jdk8u
git worktree add cvm/jdk8u jdk8u_master
```
> Branch `jdk8u/master` has many shared history nodes with `CompoundVM/master`, thus no need to duplicate the whole git history tree.

- since CVM project has got extra patches on upstream branches, we must create a CVM-jdk*u for each upstream
```
git checkout -b cvm8/jdk8u jdk8u/master
# add our patches
```

## the new cvm master
- create a new cvm master branch with no OpenJDK commit logs
```
git checkout jdk17u-target8
git checkout --orphan cvm8/master
rm -rf ADDITIONAL_LICENSE_INFO ASSEMBLY_EXCEPTION configure CONTRIBUTING.md doc make Makefile SECURITY.md src bin .jcheck
mv cvm/* ./
rmdir cvm
git add *
git commit -m 'initial load the cvm8/master branch'
```

## accomodate multiple worktrees
- Modify `cvm.mk` (now renamed to `Makefile`) to checkout the upstream branches into different worktrees at build time.
```
-jdk8u/jdk/src:
- wget -nc https://github.com/openjdk/jdk8u/archive/refs/tags/jdk8u452-ga.tar.gz
- [[ -d $(JDK8_SRCROOT) ]] || (mkdir -p $(JDK8_SRCROOT) && tar -xzf jdk8u452-ga.tar.gz -C $(JDK8_SRCROOT) --strip-components=1)
+$(JDK8_SRCROOT)/jdk/src:
+ git worktree add $(JDK8_SRCROOT) cvm8/jdk8u
+
+$(JDK17_SRCROOT)/src:
+ git worktree add $(JDK17_SRCROOT) cvm8/jdk17u
```

# Overhead
## Repository size with/without multiple remotes
About 250 MB size increase in dir .git/ after adding one jdk8u remote in current repo.

| repo content | size of .git/ |
|---|---|
| CompoundVM now (c777c7db402bb7e631221d7e63a0c6fcca47cf79) | 1226 MB |
| plus jdk8u remote | 1476 MB |

# Demo
I've created a demo in [my forked repo](https://github.com/luchsh/CompoundVM/tree/cvm8/master), steps to play with this demo:
```
git clone https://github.com/luchsh/CompoundVM.git cvm_test
cd cvm_test
make cvm8
```