PR25548: support canonicalized source-path names in debuginfod webapi

PR25548: support canonicalized source-path names in debuginfod webapi

Programs are sometimes compiled with source path names containing
noise like /./ or // or /foo/../, and these survive into DWARF.  This
code allows either raw or canonicalized pathnames in the webapi, by
letting the client pass things verbatim, and letting the server
store/accept both raw and canonicalized path names for source files.
Tests included & docs updated.

Signed-off-by: Frank Ch. Eigler <fche@redhat.com>
Reported-by: Eli Schwartz <eschwartz@archlinux.org>
Tested-by: Eli Schwartz <eschwartz@archlinux.org>