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 symlink
https://bz.mercurial-scm.org/1438
$ hg init
$ ln -s foo link
$ hg add link
$ hg ci -mbad link
$ hg rm link
$ hg ci -mok
$ hg diff -g -r 0:1 > bad.patch
$ hg up 0
1 files updated, 0 files merged, 0 files removed, 0 files unresolved
$ hg import --no-commit bad.patch
applying bad.patch
$ hg status
R link
? bad.patch