Fixesguix/guix#1975.
When using ‘read’, ‘package-field-location’ would not get source location for
atoms such as symbols, typically making it impossible to get the location of
the value of a field list (build-system gnu-build-system). This fixes that.
* guix/packages.scm (field-value-location): New procedure.
(package-field-location): Use it instead of inline code.
* tests/packages.scm ("package-field-location"): Test the ‘build-system’
field.
Reported-by: Nicolas Graves <ngraves@ngraves.fr>
Change-Id: I98c694bb6f1999fa9ca80e145fa016640067af55