Provide support for authors that want to distribute packages, by
expanding occurrences of "${foo}" in an input package spec with
the value of the 'foo' environment variable. This permits easy
configuration at install-time, e.g.,

  $ libdir=/opt/haskell/packages/lib ghc-pkg -a < NewPackage.pkg

[Clearly, a separate preprocessing pass using some other tool could
 provide identical functionality. However, the benefits to the
 package author of not having to depend on such a tool being present
 on a user's box was considered more important.
]