requirements: introduce constants for `shared` and `relshared` requirements
We add them to `WORKING_DIR_REQUIREMENTS` too as they should be stored in
`.hg/requires` and have information about the type of working copy.
Differential Revision: https://phab.mercurial-scm.org/D8926
#require docutils gettext
$ $TESTDIR/check-gendoc fr
checking for parse errors