records: Make 'report-duplicate-field-specifier' available at expansion-time.

records: Make 'report-duplicate-field-specifier' available at expansion-time.

Fixes a regression in 'guix pack -R' introduced with commit
c2dcff41c2e47f5f978f467864d5ed7829939884.  The imported modules of
'c-compiler' would be compiled in this order: first (guix records),
then (guix search-paths).  Consequently,
'report-duplicate-field-specifier' would be reported as unbound while
compiling (guix search-paths), leading to a build failure.

* guix/records.scm (report-invalid-field-specifier)
(report-duplicate-field-specifier): Move within 'eval-expand'.