Replaces assumptions proposed in #194.

LGTM! The only suggestion I have is that the new keys sv, v and vhd could be nested instead of being equal to files. For instance, something like this would make a bit more sense to me:

sources:
  # Individual source files are simple string entries:
  - src/package.sv
  - src/file1.vhd
  - src/file2.vhd

  # Source files can be grouped:
  - files:
      - src/stuff/pkg.sv
      - src/stuff/top.sv
      - sv: vendor/encrypted_sv_src.svp
      - v: vendor/encrypted_v_src.vp
      - vhd: vendor/encrypted_vhd_src.e

This should already be supported. The sv/v/vhd keys can be used in place of a sourcefile (or a sourcegroup), so even within sourcegroups we can use these keys. These sourcegroups and sourcefiles can be nested hierarchically as desired. Let me know if this doesn't work for you, or do you want to exclude the option of providing a format-specific file at the top hierarchy?

This should already be supported. The sv/v/vhd keys can be used in place of a sourcefile (or a sourcegroup), so even within sourcegroups we can use these keys. These sourcegroups and sourcefiles can be nested hierarchically as desired. Let me know if this doesn't work for you, or do you want to exclude the option of providing a format-specific file at the top hierarchy?

Ah no that's already enough. I think I got the impression from the code that sv etc. are on the same level as files. But in that case LGTM!