internal

As a way to provide a more useful way to understand what's changed
between exports of Renovate data - via `renovate-graph` or our other
supported formats, we should introduce a separate cli,
`renovate-packagedata-diff`.

This could be used to i.e.:

- see what changes are coming in as part of a PR/MR
- summarise changes in `git diff` outputs for projects that source-track
  their `renovate-graph` exports
- wire into CI to i.e. raise warnings when changing important
  dependencies

It makes sense to be part of dependency-management-data, rather than
`renovate-graph`, as we already have some good understanding of the
structure here, and can provide a more convenient output format.

Additionally, producing a Go binary will be more convenient i.e. for CI
builds.

This should - at least medium term - also produce a stable JSON response
output, which can be used for scripting.

For now, we will only provide a human-readable summary.

At some point, we may also expose this as part of a Go API.

To do this, we need to:

- create a new CLI
  - which can provide `--debug` logging
- present the output to the user, in a tabular, colourised fashion
- handle being called ad-hoc or as a `GIT_EXTERNAL_DIFF` tool
- create a new `CompareFiles` function, which orchestrates the overall
  diffing between two package dumps
- perform the difficult job of determining which modifications are
  actually modifications, and not presenting as addition/deletion
- wire in the command to `gendoc`

Note that there is a fair bit of inline code commentary, and areas which
look a little "spooky" but don't have comments could be that in the time
it's taken to finish implementation have since been forgotten, or test
cases lost.

There are test cases in the -example project, under
`/renovate-packagedata-diff`.

Closes #616.