Create release source artifact containing all submodules

Testing:

```
./build.py release-src
```

This will create a source release.  If the repo is tagged, you should get the correct release version here.

The resulting zip file should contain
- all submodules
- correct executable flags on all files executable in the git tree
- `components/antlib/resources/gitversion.xml` containing the release version numbers

The unpacked zip file should build all the same targets as for the git repo, the only exception being that the release-src target will not work, so `./build.py release-all` will exclude the source zip when run outside git.  We could remove this from release-all so that it's only for binary artifacts.