Streamline RPM generation process

@aiuto in the [review for #179](https://github.com/bazelbuild/rules_pkg/pull/179#discussion_r438335224) pointed out that there were several confusing parts in the consolidated RPM generator.

Instead, the process could be streamlined to something that looks like this ([see my comment to that same review](https://github.com/bazelbuild/rules_pkg/pull/179#discussion_r464526616)):

> 1) The template spec file is expanded to a more fully-qualified spec file that can be invoked by `rpmbuild` with minimal additional magic. Also, preferably with a single template expansion scheme.
> 2) The rpmbuild directory structure is created, and, if necessary (see below), the already-built artifacts are put in the right place.
> 3) `rpmbuild` is invoked with the necessary arguments to build the RPM (This would be `make_rpm.py`)
>
> This could help us dramatically reduce the complexity of `make_rpm.py`.
>
> We might be able to somewhat reduce the number of file copies this process entails by having the `%install` phase simply copy (or hardlink) the files from the bazel build tree to where they are supposed to be relative to RPM builds the package. Haven't yet tried this out either.

This should result in much more readable and efficient code overall.