task: detangle build-time and runtime dependencies

There are a few interconnected issues with how we use bundler for both the build chain and the plugin manager. This is not an exhaustive list.

- The runtime and development dependencies of logstash-core are somewhat arbitrarily split between `Gemfile.template` and `logstash-core/logstash-core.gemspec`
- use `gemspec path: 'logstash-core'` in the `Gemfile.template` instead of making `logstash-core` a runtime dependency; this ensures that the `add_development_dependency` declarations in `logstash-core/logstash-core.gemspec` are effectively development dependencies of the `Gemfile.template`
- move non-plugin (a) runtime dependency declarations and (b) development declarations from `Gemfile.template` to the gemspec
- NOTE: version constraints for plugins must be in `Gemfile.template` in order to support plugin upgrades in shipped artifacts
- temporary pins of _transitive_ dependencies can continue to live in `Gemfile.template`
- `bin/logstash-plugin install --development` installs not only the development dependencies of logstash core, but also of _each of the plugins that are currently installed_, and it does not _merge_ the requirements so that the last one wins (even if it overrides a _runtime_ requirement that is in the checked-in `logstash-core.gemspec` or `Gemfile.template`).
- do we ever _actually_ need the development dependencies of all of the plugins while developing Logstash core? Why aren't the dev dependencies we need specified as development dependencies of logstash core?
- Because we build artifacts directly from source, the dependencies that are required for building the artifacts but _not_ required for the built artifact itself are a part of the shipped bundle (e.g., `rake`, `octokit`, `childprocess`, `minitar`, `fpm`).
- I _believe_ this can be solved by using an intermediate staging directory and installing its bundle using its plugin manager, at which point the `build` group of dependencies in the `Gemfile.template` could be made development dependencies in `logstash-core/logstash-core.gemspec`, which would ensure that they're never installed in the built artifact.