loaddefs-gen: Group results by absolute file name

loaddefs-gen: Group results by absolute file name

loaddefs-generate produced an incomplete output file if 1) it
was called with a relative file name and 2) that same file was
specified via a generated-autoload-file cookie in a subset of
the input files.  In that case, autoload entries were lost
because loaddefs-generate writes the same output file twice:
once for the relative name specified by the caller and once
for the absolute name that loaddefs-generate--parse-file
returns for the generated-autoload-file value.
This has been fixed.  (Bug#60318)

* lisp/emacs-lisp/loaddefs-gen.el (loaddefs-generate): Expand file
names when grouping loaddef files.