The @extensions slot may be stored either as a named character vector of
requirement strings (the legacy/unversioned form) or as a named list whose
entries are length-2 character vectors c(requirement = ..., version = ...).
This helper returns the requirement strings as a plain named character
vector, which is the form used for dispatch and suffix comparison.
extensionRequirements(ext)A named character vector of requirement strings.
The contents of an @extensions slot (character vector or list).