Use the json schema as input for templating

[wagoodman]

The template functionality today uses the go struct fields from the syft/format/syftjson/model directory as input, which means that in order to write a template you need to read the syft source code. For instance:

"Package","Version Installed", "Found by"
{{- range .Artifacts}}
"{{.Name}}","{{.Version}}","{{.FoundBy}}"
{{- end}}

• Artifacts approximates to artifacts in the json schema
• Name approximates to name in the json schema
• Version approximates to version in the json schema
• FoundBy approximates to foundBy in the json schema

This change adapts the template input to require the same fields as what you will find in the JSON output of syft (exactly what's in the json schema):

"Package","Version Installed", "Found by"
{{- range .artifacts}}
"{{.name}}","{{.version}}","{{.foundBy}}"
{{- end}}

Caveats are that struct helpers cannot be used on maps, which is what ultimately is used for templating. This means that some alterations will need to be made, for example replacing hasField for index:

2,3c2,3
< {{- range .Artifacts}}
< "{{.Name}}","{{.Version}}","{{.FoundBy}}","{{ if hasField .Metadata "Author" }}{{.Metadata.Author}}{{ else }}NO AUTHOR SUPPLIED{{end}}"
---
> {{- range .artifacts}}
> "{{.name}}","{{.version}}","{{.foundBy}}","{{ if index .metadata "author" }}{{.metadata.author}}{{ else }}NO AUTHOR SUPPLIED{{end}}"

Since this will impact all templates a new SYFT_FORMAT_TEMPLATE_LEGACY/format.template.legacy option has been provided that, when enabled, will use the original struct-approach.