Expose CcSkylarkApiProvider as a first-class provider in Skylark

[mboes]

Description of the feature request

It's possible to write Skylark rules that have cc_* rules as dependencies. Via the CcSkylarkApiProvider, it's possible for these Skylark rules to e.g. collect the list of transitive header files from these dependencies. But,

1. it's not possible to write a Skylark rule that itself provides CcSkylarkApiProvider,
2. it's not possible to test whether a dependency provides CcSkylarkApiProvider.

A workaround for (2) is:

[dep for dep in ctx.attr.deps if hasattr(dep, "cc")]

where one would expect to be able to use:

[dep for dep in ctx.attr.deps if CcSkylarkApiProvider in dep]

Underlying both (1) and (2) is the fact that unlike say DefaultInfo and OutputGroupInfo, CcSkylarkApiProvider, along with many other native providers, are not exported to Skylark as a global.

My use case for (1) is writing a replacement for cc_import in Skylark, since that rule does not currently provide CcSkylarkApiProvider.

Environment info

• Bazel version (output of bazel info release): 0.9.0

Have you found anything relevant by searching the web?

I've seen mention of "Bazel sandwiches" where is some language. e.g. https://blog.bazel.build/2017/03/07/java-sandwich.html. It looks like for Java at least, the equivalent of (2) above is possible. Though the blog post mentions that the equivalent of (1) is still not possible. The Java functionality is provided via the java_common struct. For C/C++, the corresponding struct cc_common does not expose the corresponding methods.

Also related: #2163

Anything else, information or logs or outputs that would be helpful?

(If they are large, please upload as attachment or provide link).

[hlopko]

We are currently working on a C++ sandwich. CcSkylarkApiProvider will very likely be replaced by some other thing and we'll also include corresponding cc_common functionality. We will provide a design doc for public review in Q1. Assigning to Pedro since he leads this effort.

[oquenchil]

Sandwich is ready to be used. Closing this bug:
https://docs.bazel.build/versions/master/skylark/lib/CcInfo.html