Work with non-serializable plugin settings

[jasonkuhrt]

Description

• Change backingTypesMap to mapping graphql-nexus/nexus#782 (comment)
• plugin settings can contain non-serializable data
• like functions or objects with circular references
• when we refactored the reflection system one of the things we did was make it safer by never running the app in the main cli process. So for example an infinite loop or process exit within the user's app would not bring down the main cli process
• however reflecting on used plugins this way (aka. gathering used plugin manifests) results in the settings of those plugins needing to be serializable
• and that is not something we intended
• the only solution after discussion with @Weakky is that we need to reflect on used plugins in process.
• this exposes the main cli process to the above-mentioned risks
• dealing with those risks will be treated as a new feature

CC @kristoferma