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Accessing undefined variables signals an error in src/project/*.lisp

fixes #1892

• src/project/protocol.lisp (lookup): added if-undefined keyword
  parameter; adapted documentation string
  (setf lookup): similar
  (value): added optional default parameter
  (lookup t t): accept but ignore if-undefined keyword argument
  (lookup :around t t): accept and process if-undefined keyword argument
  (value t t): accept and process optional default argument
  (platform-requires t cons): supply default value to `value' call
  instead of ignoring errors
• src/project/mixins.lisp (header): updated copyright
  (instantiate? conditional-mixin t): supply default value to `value'
  call instead of ignoring errors
  (setf lookup t direct-variables-mixin t): accept but ignore
  if-undefined keyword argument
• src/project/aspect.lisp (make-aspect-class-form): supply default value
  in `value' call
• src/project/aspects.lisp
  (builder-constraints aspect-builder-defining-mixin t): similar
  (define-aspect shell): shell.command must be defined but may be nil
• src/project/classes-spec.lisp
  (shared-initialize :after version-spec t): likewise
  (requires version-spec): likewise
  (provides version-spec): likewise
  (check-access version-spec t): likewise
• src/project/classes-model.lisp (add-dependencies! job job-spec):
  likewise
  (deploy job): likewise
  (deploy-dependencies job): likewise
• src/project/json.lisp (load-project-spec/json-1): initialize name slot
  of the version-spec early to allow early access to the direct
  variables of the object
• src/commandline-interface/main.lisp (analyze-project): supply default
  value to `value' call instead of ignoring errors
  (load-projects/versioned): likewise
  (configure-jobs): likewise
  (configure-distribution): likewise