zip: add `#:path-prefix` argument
scribble: fix `history` spacing
scribble/decode: document treatment of whitespace by `decode-content`
raco pkg install: fix tmp-directory clean-up

Fix when installing from a non-directory in `--source` or
`--binary` mode, and fix clean up of a GitHub-generated archive
when using a path within the archive.
raco pkg install: change interpretation of archive with a single directory

When an archive used as a package source has all of its content
within a single top-level directory, then treat that directory's
content as the package content, instead of treating the directory
itself as the package content.

The change makes "x.zip" created with

  zip -r x.zip x

work the same as "x" as a package source. It also makes a ".zip"
for a package's GitHub repository (from clicking the "Download ZIP"
button) work the same as the repository.

This change is backward incompatible, but it's unlikely to break
any working package (since that would be a single-collection package
that provides only a subcollection).