libintl, iconv, gettext: account for libc provider and externals (#35450)

* libiconv can be provided by libc, so update packages which depend on
  libiconv to require the iconv virtual instead
* Many packages need special consideration when locating iconv depending
  on whether it is provided by libc (no prefix provided) or the libiconv
  package (in that case we want to provide a prefix)
* It was also noticed that when an iconv external was provided, that
  there was interference with linking (this should generally be handled
  by Spack's compiler wrappers and bears further investigation)
* Like iconv, libintl can be provided by libc or another package, namely
  gettext. It is not converted to a provider like libiconv because it
  provides additional routines. The logic is similar to that of iconv
  but instead of checking the provider, we check whether the gettext
  installation includes libintl.

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