Add pkg_sub_rpm rule for RPM subpackages (#824)

* rpm: Add support for sub packages to make_rpm.py script

Before we can enable support for sub RPM building as part of a single
`pkg_rpm()` rule we must add the underlying support to make_pkg.py
which is the underlying driver for `pkg_rpm()`.

This covers three pieces:

 * specifying `buildsubdir` rpm variable
 * capturing multiple RPM files as outputs
 * injecting sub RPM definitions into specfile

* rpm: Factor out rpm_ctx helper

The various processing functions pass around a bunch of collections
everywhere which is a bit fragile.  This collects them together into a
struct to make it a bit less messy.

* rpm: Factor out _process_dep() helper function

_process_dep() handles processing an individual dep and is currently
called from the processing loop.  We'll need to re-use this logic for
processing individual sub RPMs as well so we want it in a helper.

* rpm: Capture generated output RPM files in rpm_ctx

Currently we only generate one RPM file, but once we generate sub RPM
files we'll need to be able to capture those outputs as well.  This
prepares us for that step.

* rpm: Add args for make_rpm to rpm_ctx

We'll need to add additional arguments to make_rpm for sub RPM
building.  It's easier to capture this in our context object than to
try to shuttle these bits around.

* rpm: Pass correct `--name` argument to make_rpm

If we don't pass the correct RPM name to `make_rpm.py` than we won't be
able to correctly determine the subrpm names.  Currently, the name is
only used by `make_rpm` to generate some progress output, so this
shouldn't break anything internally.

* rpm: Implementation of `pkg_sub_rpm` rule

This introduces a `pkg_sub_rpm` rule that allows us to generate and
capture RPM subpackages and generate them as part of a single RPM
invocation in lieu of cobbling this together from multiple RPM rules.
This has a few benefits:

- faster execution due to single rpmbuild invocation
- sharing configuration between RPMs in the same fashion as vanilla
  RPM building from a specfile
- will enable the proper construction of debuginfo RPMs in a later PR

The current implementation *only* works with non-specfile based rules
and currently allows for a subset of the general RPM configuration.

Internally, the process is for `pkg_sub_rpm` to generate a
PackageSubRPMInfo provider that will subsequently get consumed by the
`pkg_rpm` rule that generates the actual RPMs.  We re-use the internal
dependency processing logic that's used by the top-level RPM to
generate the content related to the sub-RPM.

* rpm: Update entry points for RPM rules to include pkg_sub_rpm

This change updates `rpm.bzl` in two ways to account for sub RPMs:

- it adds an entrypoint for `pkg_sub_rpm`

- it adds a check in `pkg_rpm` to assert incompatibility with
  `spec_file`

* examples: Add an example of how to use the subrpm rule

This provides a basic example using the `pkg_sub_rpm` rule to generate
multiple RPMs.

* Fix buildifier noise

* Fix make_rpm failures

* doc: Clean up sub-RPM docstring and add to doc_build

The initial docstring for pkg_sub_rpm is not great, so this change
fixes that while adding this to the list of rules to have
documentation generated for them.

* doc: Additional documentation for subrpms in pkg_rpm

This clarifies the `subrpms` attribute usage as well as indicating the
incompatibility with `spec_file` mode.

* Fix issue in subrpm passthrough

When adding the check to verify if we can use subrpms, this pass
through wasn't added.

* Add a basic test for pkg_sub_rpm

This introduces a basic test with a single sub RPM and main RPM each
containing a single source file.

* Tweaks to test

* Test fixes for CentOS 7

The `rpm` version on CentOS 7 doesn't work exactly the same way as
newer versions of rpm and requires a `-p` parameter to inspect
non-installed RPMs.  CentOS 7 also inserts a `Relocations` field that
we didn't see on other platforms so we'll filter that out to make our
test a bit more portable.

* Move PackageSubRPMInfoProvider into rpm_pfg.bzl

This is private to the RPM rules so should probably live there.