Update Process.dependencies to allow more flexibility when setting envs

[kylewlacy]

This PR updates how the dependencies field is use when a process recipe is baked. Each dependency recipe can contain a brioche-env.d/env directory to set env vars on the process, but previously this was limited to appending $PATH-like env vars: each entry in brioche-env.d/env had to be a directory (whose name was an env var), where each entry within was a symlink, whose target would be appended to the env var (separated by :).

This PR lifts the restriction, and now each env var within brioche-env.d/env can either be a file, directory, or symlink:

• Directory: Same behavior as before. Each entry within the directory must be a symlink whose target is appended to the env var (the name of the symlink doesn't matter)
• Symlink: If the env var is empty, then it will be set to the target of the symlink. This is only used as a fallback, so this will be skipped if the env var is already set
• File: If the env var is empty, then it will be set to the file's contents. This is only used as a fallback, so this will be skipped if the env var is already set

So, this is now a valid structure for a dependency:

- bin/
  - cc
- brioche-env.d/
  - env/
    - PATH/
      - path1 -> ../../../bin
    - CC -> ../../bin/cc
    - DEBUG [file containing the string "1"]

When used as a dependency, it will append bin to $PATH, set $CC to the path to bin/cc (if unset), and set $DEBUG to 1 (if unset)