Rename plugin semantics

[tonyketcham]

We should follow a standard pattern for naming of the plugins and their exports. These exports should also be named rather than exported as default so that a standard name is used in our documentation and ecosystem consumption of the plugin

• markdownTransformer exports transformMarkdown()
• resolverSvimg exports resolveSvimg()
• providerApollo exports provideApollo()
• sourceFilesystem exports sourceFilesystem()