Split v1-5 DB distribution concerns to a new legacy package

[wagoodman]

With grype DB v6 we'll be tracking all distribution concerns with the schema instead of something that is shared across schemas (e.g. DB curator, listing file shape, metadata file shape, etc). This issue is just in place to migrate existing code to make room for the new schema and distribution concerns.

Todays layout:

grype/db
├── v1/
├── v2/
├── v3/
├── v4/
├── v5
│   └── ...
├── curator.go           # used to search for and download DBs
├── listing.go           # listing.json definition for distribution
├── metadata.go          # metadata.json definition for distribution
├── status.go            # db status command support
└──*_provider.go         # higher-level DB access helpers (provides GetByDistro, GetByCPE, GetByLanguage, etc)

Proposed layout:

grype/db
├── legacy
│   └── distribution                    # support v1-5 distribution concerns
│       ├── curator.go
│       ├── listing.go
│       ├── metadata.go
│       └── status.go
├── v1/
├── v2/
├── v3/
├── v4/
├── v5/
└── v6/
    ├── distribution                # future code...
    │   └── ...
    ├── installation                # future code...
    │   └── ...
    └── ...

This will mean updating grype-db to additionally use these updated paths.