`plan` command should validate that a user has specified input files properly

[rspurgeon]

The current behavior begins, apprently, to read the files in the current directory.

kongctl plan ./docs/examples/declarative/control-plane/basic.yaml
Error: failed to load configuration: failed to parse .golangci.yml: unknown field 'issues' in .golangci.yml. Please check the field name against the schema

where that file is local to the directory but that is clearly not the user's intent.