gexp: 'local-file' resolves relative file names.

gexp: 'local-file' resolves relative file names.

* guix/gexp.scm (<local-file>): Rename constructor to '%%local-file'.
Add 'absolute' field.
(%local-file, extract-directory, absolute-file-name): New procedures.
(current-source-directory): New macro.
(local-file): Adjust call to '%local-file'.
(local-file-absolute-file-name): New procedure.
(local-file-compiler): Force the 'absolute' field.
* tests/guix-system.sh: Test whether 'local-file' canonicalization
works.
* doc/guix.texi (G-Expressions): Adjust.