Support in-tree podspecs

This enables people to use local podspecs as the source of truth for target
definition and build settings instead of a BUILD file.

It introduces a single `repository_rule` to generates a BUILD files for
*every* podspec in the source tree _adjacent_ to the podspec.

### Example `repository_rule`

```
# WORKSPACE
gen_pod_build_files()
```

Considerations:

- the rule must be a repository rule, to fit into bazels [evaluation
model](https://github.com/bazelbuild/bazel/blob/master/site/docs/skylark/concepts.md#evaluation-model)

- it may glob all the *.podspecs as an input and use these inputs for cache
invalidation

- the `BUILD` file compiler needs to be updated to manage the local podspecs

- and @ import support needs to be updated to hmaps or symlinking to
the right directory.