Add `git_module` source plugin

Port of bst-external!126 (merged)

git_tag supports submodules. But there are some limitations.

• There is no support for explicit recursive submodules. The only way to support that was to use multiple git_tag sources. But that needed manual tracking of modules. Now, we can add a module to a module, and tracking properly considers the right module commit.

• There was no explicit information on the commit. That made it more complex for third party tools to analyze source code of a BuildStream project. Now the commit is stored during tracking.