Makefile change
Provide additional identifiers needed for pkg-index.
Revert "Makefile change"

This reverts commit e0a9a92c0f9f8f3a2a66fa38d9786f7ccdf6fbd9.
fix over-eager shortcut in the implementaiton of continuation jumps

Don't jump past a prompt when jumping to a continuation that is
a prefix of the current one.

Reported by Max New
Split almost everything else from the main repository.

The source to the split packages is in repositories under the
`racket` organization on GitHub. The repositories are all named
according to the pkg name, except for multiple-package
repositories such as `racket/compiler` which is named based on the
old directory name without the `-pkgs` suffix. Thus

   `pkgs/compiler-pkgs` -> https://github.com/racket/compiler

The Makefile has also been adjusted to pull packages from the
catalog when you type `make`. This currently relies on some tricks
that will break if you try to specify a particular set of `PKGS` on
the command line. We plan to improve this soon.

The packages in `pkgs/racket-pkgs` and `pkgs/base` are staying in
the repository, since they logically belong with the core code.

The `plt-services` package is still in the repository, but will
move out soon.
Ignore extra-pkgs/
package manager: adjust parsing of file:// URLs

For some reason, I previously made the package manager parse file URLs
by stripping away "file://" and treating the rest directly as a
path. Maybe it was to support relative paths, or maybe it was to
implicitly disallow query and fragment parts of the URL, but it seems
like a terrible idea; I've switched to `string->url` plus `url->path`.

As a result, parsing now implicitly allows and ignores query and
fragment parts of a "file://" URL. That's intended as a feature,
allowing extra information to be attached to a path in, for example,
a catalog.
package manager: "file://" URLs and "?type=..." queries

Allow a "file://" URL to specify a type that causes the path
to be installed as a link or static link. A type query like
that is mainly intended for use in a catalog, where a catalog
of local directories could create links as needed for other
packages (that might be pulled from other catalogs).
remove "native-pkgs"

Treat native-library packages like all the others that are now
served from the catalog.
make: link packages via local catalog

Change the way that packages in "pkgs" are handled by `make`:
create a catalog that causes them to be installed on demand
as directory links.
fix build for a fresh checkout
Fix Travis tests by using collection paths instead for file paths.
make: fix Windows build
make: clear out "local" installer mode

The "local" installer mode made sense only with a "pkgs"
directory that had everything to be included in the installer.
A file for this already existed.
make: fix Unix-style build
raco pkg: fix removal before re-instal of extra clone-sharing packages
raco pkg: fix a test
correct inverted sentences
Enable some extra tests in Travis CI.