Gauche manages extra libraries and extension modules as packages.
Each package source tree has package.scm on top directory, which
define-gauche-package form that provides metainformation
about the package—the package name, veirson, author, dependencies, etc.
When the package is installed, the standard installation process copies
that information, with additional information such as the version of
Gauche used to build the package, into .packages subdirectory
of the library installation path, with the name PACKAGENAME.gpd,
PACKAGENAME is the name of the package.
We collectively call package.scm and *.gpd as package description file.
This module provides utility procedures to read and write package description files, and search installed *.gpd files.