index Paths and Paths_ (resp. for PackageInfo)

These can currently be found by searching the documentation for
`Paths_pkgname` and `PackageInfo_pkgname`, but you pretty much
need to know that to begin with to find them. Add Sphinx index
entries to make them more discoverable.

(cherry picked from commit d51c554)

doc/cabal-package-description-file.rst

@@ -2965,6 +2965,9 @@ Right now :pkg-field:`executable:main-is` modules are not supported on
 Accessing data files from package code
 --------------------------------------
 
+.. index:: Paths
+.. index:: Paths_
+
 The placement on the target system of files listed in
 the :pkg-field:`data-files` field varies between systems, and in some cases
 one can even move packages around after installation
@@ -3020,6 +3023,9 @@ the configured data directory for ``pretty-show`` is controlled with the
 Accessing the package version
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
+.. index:: PackageInfo
+.. index:: PackageInfo_
+
 The auto generated :file:`PackageInfo_{pkgname}` module exports the constant
 ``version ::`` `Version <http://hackage.haskell.org/package/base/docs/Data-Version.html>`__
 which is defined as the version of your package as specified in the