gen_block: Resolve dependencies.

[inferiorhumanorgans]

This will:

• Assume each YAML file has exactly one block
• Assume he name of the YAML file is the unsanitized block name
• Load additional YAML files as needed
• Only garbage collect extra blocks (not other types).

This enables patterns like this:

block/PORT_ELC:
  description: Port (w/ Event Link) Control Registers.
  extends: PORT
  items:
  - name: EIDR
    description: Event input data register.
    byte_offset: 4
    access: Read
    bit_size: 16
    fieldset: regs::EIDR
fieldset/regs::EIDR:
  description: Event input data register.
  bit_size: 16
  fields:
  - name: EIDR
    description: Pmn Event Input Data.
    bit_offset: 0
    bit_size: 16
    enum: vals::EIDR_EIDR
enum/vals::EIDR_EIDR:
  bit_size: 16
  variants:
  - name: '0'
    description: Low input.
    value: 0
  - name: '1'
    description: High input.
    value: 1