Use conda-style selectors rather than platform keys to specify deps

Currently in the metapackage YAML files we use keys to specify alternate names for packages for different platforms, like this:

deps:
  ciecplib:
    conda:
    deb: ciecp-utils
    rpm: ciecp-utils
  ecp-cookie-init:
  htgettoken:
  kerberos:
    conda: krb5
    deb: krb5-user
    rpm: krb5-workstation
  ligo-proxy-utils:

Would it be easier to maintain the metapackages if we used conda meta.yaml-style selectors for this, like this:

deps:
  - ciecplib  # [conda]
  - ciecplib-utils  # [deb or rpm]
  - ecp-cookie-init
  - htgettoken
  - krb5  # [conda]
  - krb5-user  # [deb]
  - krb5-workstation  # [rpm]
  - ligo-proxy-utils

cc @adam-mercer