Log Message:
converters/pandoc: Turn the package into a meta package

Background: The upstream package, pandoc, historically contained the CLI
tool but it was split into two separate packages: pandoc and
pandoc-cli. When that happened I (pho@) renamed the original
converters/pandoc to converters/hs-pandoc-base to install the Pandoc
library, and replaced the contents of converters/pandoc with the upstream
pandoc-cli.

The intention was to make the change less surprising to our users. That is,
I didn't want the CLI tool to suddenly disappear from
converters/pandoc. But it complicated matters because the upstream
pandoc-cli used a distinct versioning scheme from pandoc. As a result I was
forced to use a weird versioning scheme for converters/pandoc, whose
PKGVERSION actually tracked the version of converters/hs-pandoc-base,
despite its contents being that of pandoc-cli. The difference in package
name, upstream "pandoc" vs. pkgsrc "hs-pandoc-base", was \ 
also confusing.

So I:

* renamed converters/hs-pandoc-base to converters/hs-pandoc, which tracks
  upstream "pandoc",
* created converters/pandoc-cli, which tracks upsream "pandoc-cli",
* and turned converters/pandoc into a singleton meta-package which depends
  only on converters/pandoc-cli

so that we can eliminate the weird versioning scheme and can still provide
the CLI tool (indirectly) from converters/pandoc.