remove dead code

The removed line was added due to a misreading of a failure
path as a slow path.

Thanks to Jon Zeppieri.
Move `explode-path' from `racket/path' to `racket/base'

The revised `explode-path' runs in time proportional to the
length of the path, instead of quadratic in the number of
path elements. The difference doesn't matter much in my
program, but I'm reluctant to leaving the implementation
as quadratic (which is forced by using `split-path').
render non-installaction-scoped package docs with an indirection

The indirection uses a "local-redirect.js" script to rewrite
the document links to local links within the browser. This mechanism
is a step towards distributing compiled packages that include
already-built documentation, where paths to other documentation
can be different than in the build environment.

If the links are not rewritten, they are queries to
"pkg-docs.racket-lang.org", with the idea tha such a server will exist
for reading all package documentation online. Also, a package's
documentation that refer to documentation for uninstalled packages,
in which case the corresponding links will not get rewritten and
will continue to point to the server.

Rendering the "local-redirect.js" script spends a lot of time just
converting among different path formats. Various library changes in
this commit are aimed at speed up those conversions, but the big
improvement came from a `path->url-string' that shortcuts conversion
os simple Unix paths.
raco pkg: "indices" -> "catalogs" in dependency message