Add `pdfbaker validate`

[dannyadair]

Takes config file argument like bake.
Performs a quick validation to confirm that all requirements are met:

• config found
• config syntactically valid
• config has required keys (documents)
• directories of documents (as listed) exist
• document configs found
• document configs syntactically valid
• document configs have required keys (filename, pages) - should "filename" maybe be changed to "name"?
• pages found as configured (directory, config)
• page configs syntactically valid
• page config has required keys (name) - should "name" maybe be changed to "filename"?
• if images are listed, images exist (going too far?)
• templates found as configured
• validate template content? (defusedxml check well formed, root is ) (going too far?)
• Inkscape installed if required by config
• Ghostscript installed if required by config
• build directory/directories already exist (warning)
• dist directory/directories already exist (warning)

Put this in a separate module.
Always run this first before attempting bake. (still good for the user to have this separate command)

[dannyadair]

Updated checklist to include build/dist already existing. This covers the concerns of #21 which I'm cancelling in favour of a better validate.

[dannyadair]

Should also be able to limit the document configs to check (like bake)

[dannyadair]

Hmm, or maybe just bake --dry-run?
The existence of build/dir would become a warning log in a dry run (debug log otherwise).
Might need to pull some checks up (like checking for ghostscript before trying to use it? Or just pass dry_run all the way through everything? If we don't create files the regular ghostscript command will fail for a different reason...)
But yeah, there should be a way to avoid duplication.

[dannyadair]

The existence of build/dir would become a warning log in a dry run (debug log otherwise).

Or maybe better explicit --warn-if-exists and --fail-if-exists.
This can become very useful when e.g. running on a server via cronjob.

• The default "build" and "dist" are common naming conventions and people should know to not be calling their documents that.
• An existing dist directory is expected on re-runs, so checking - not warning - should be optional. Having to write more is just the price then.

The "build" directory is a little more concerning because it gets deleted (and everything it contains, as you rename your pages we don't just delete the files that were/would be created in this run). Maybe the default build directory should be in /tmp (and its location quoted when kept).

[dannyadair]

Ok I will go with --dry-run and close this issue in favor of #30