raco pkg: don't treat "info.rkt" as a conflict

Even though "info.rkt" files are in collections, `raco setup'
treats them in a directory-specific way --- and that's necessary
for specifying things like `compile-omit-files' in a collection
splice.
libpng adjustment

Keep error if no "libpng" is found.
scribble/base: handle special characters in URLs

For characters in the URL that give LaTeX trouble, convert
them to % encoding.
scribble HTML rendering: unreserved-encode "on this page" links

In other words, replace "(...)" with "%28...%29" in links.
raco pkg create: support "source" and "binary" bundling

Adds `--from-dir' and `--from-install' flags to select the interpretation
of the argument as a directory or as the name of an installed package.
Relevant to PR 13669

Adds `--as-is' (the default), `--source', and `--binary' flags to
select a pruning mode.

The `raco setup' tool recognizes a `rendered-scribblings' specification
in "info.rkt" to trigger moving rendered documentation into place,
registering its tags in the cross-reference database, and fixing up
references to "local-redirect.js"; the presence of a "synced.rktd"
indicates when those fixups have been performed (since, if the package
is installed in a user-specific scope, the documentation doesn't actually
move anywhere). Finally, "out<n>.sxref" needs to report paths relative to
the documentation's directory, and then the relative-directory references
need to be suitably resolved at derserialization; some support for such
relative paths was in place, but it wasn't quite general enough before.