link development-mode packages in a separate pkg database

Use `PKGS' with the top-level makefile to select packages for
a development-mode build. The default is "main-distribution",
which includes all packages in "pkgs".
setup/infotab: drop `(require string-constants)' and `(string-constant <id>)'

The problem with allowing `(require string-constants)' in an
"info.rkt" file is that "string-constants-lib" is a package that might
not be installed.  Removing it allows any "info.rkt" file to be loaded
with just the core.

Strings constants were used for `textbook-pls' and
`drscheme-language-positions' entries. To enable access to string
constants for those specifications, DrRacket now recognizes
`get-textbook-pls' and `get-drscheme-language-positions' as
module path plus symbol specifications.

Also, fix up conflict checking in `raco pkg install' so that ".zo"
files without source do not count as provided modules unless an
"info.rkt" declares `assume-virtual-sources' (which is where we
got into trouble, again, with `string-constants'), making it
consistent with the way that `raco setup' removes ".zo" files.
raco pkg: fix dependency checking for user and shared scopes