Add validate_dependencies flag to pipeline upload

[dabarrell]

Description

Adds a --validate-dependencies flag to the pipeline upload command, which will fail the upload if any steps depend on keys that don't exist.

Context

Changes

❯ ./buildkite-agent-local pipeline upload --help
Usage:

    buildkite-agent pipeline upload [file] [options...]

<snip>

Options:

<snip>

  --validate-dependencies      When true, validates the dependencies of each step when uploading. [$BUILDKITE_PIPELINE_VALIDATE_DEPENDENCIES]

Testing

• [x] Tests have run locally (with go test ./...). Buildkite employees may check this if the pipeline has run automatically.
• [x] Code is formatted (with go fmt ./...)