Validate mixins

[natalieparellano]

Resolves #578

[natalieparellano]

If you set aside the platform -> common changes (which I’m not in love with, because things like platform.LayersMetadata read better than common.LayersMetadata) the most substantive changes are -

detector.go - MixinValidator interface
platform/common/analyzed.go - moved structs for analyzed.toml here, added AnalyzedMetadata and AnalyzedMetadataBuilder interfaces
platform/pre06/analyzed.go - lots of boilerplate code to implement DecodeAnalyzedMetadataFile and NewAnalyzedMetadataBuilder
platform/v07/analyzed.go - more boilerplate for the new api
platform/common/exit.go - moved codes here to avoid circular dependency
stack.go & stack_test.go - stack validation logic

I actually like how the code reads. My main concern is that it's an awful lot of code to write (for just analyzed.toml). I would love to hear what others think.

[natalieparellano]

Received some good feedback in sub team sync - will take a stab at making this simpler.

[natalieparellano]

Given that this validation is no longer needed, I'm going to close this PR.