Rework of CLI flag remapping

[dansimau]

This reworks the flags feature and refactors a lot of the code under the hood
to change the design and fix some issues.

• Fix the --help issue.

  Previously, --help didn't work at all and resulted in an error: pflag: help requested (see --help prints "pflag: help requested" #1). This fixes that issue and also improves the printing of
  user flags in the --help text. User flags are now shown at the bottom,
  separate from builtin and global astro flags. E.g.:

  $ astro plan --help
  Generate execution plans for modules
  
  Usage:
    astro plan [flags] [-- [Terraform argument]...]
  
  Flags:
        --detach           disconnect remote state before planning
    -h, --help             help for plan
        --modules string   list of modules to plan
  
  Global Flags:
        --config string   config file
        --trace           trace output
    -v, --verbose         verbose output
  
  User flags:
        --env string      Environment to deploy
        --region string   Region to deploy
  $
  

• Previously, Terraform variable names were remapped to a different flag name
  by specifying the flag: option in the variable config, e.g.:

  - name: app
    path: core/app
    variables:
    - name: aws_region
      flag: region
  

  This meant if you had many modules defined with the same variable, you had to
  type out the mapping correctly every time or else you would get an error.

  The mapping configuration has now moved to its own block in the project
  config, which now looks like:

  flags:
    aws_region:
      name: region
      description: Region to deploy
  

  Users can now also specify a description that will appear in the --help text.

  Note that the flags section is completely optional, and be default astro will
  just expose the full variable name as the CLI flag.

• Refactoring

  The implementation of user/project flags did a lot of the error checking in
  the cli/astro package itself, however, we want to move in a direction where
  those checks occur in the core astro package. The CLI should be a thin
  wrapper around the core package.

  Because this results in a breaking change in the config, I'm going to bump
  this to 0.5.0.

[dansimau]

@DragonZ I'm working on the tests now, but please go ahead and do a first pass. Thank you 🙏