refactor: refactor depviz

[ahamidullah]

Major change is separation of package main into pkg/issues and pkg/airtabledb.
Also some general code cleanup, commenting, etc.

┆Issue is synchronized with this Trello card

[guardrails]

⚠️ We detected security issues in this pull request:

Insecure Use of Dangerous Function (1)

• /cli/cmd_web.go:122

More info on how to fix Insecure Use of Dangerous Function in Go.

[moul]

As depviz is a monobinary, I prefer to keep the main at the root of the directory

./cmd/... is suggested for "ecosystem repositories" that contains multiple mains and that are primarily used as golang libraries

Depviz is an application first that is more often downloaded as is than used as packages

[moul]

the package airtableDB should be named airtabledb to avoid case sensitive issues with package managers etc

[moul]

LGTM