Return a vector describing the package in the current buffer.
The vector has the form
[FILENAME REQUIRES DESCRIPTION VERSION COMMENTARY]
FILENAME is the file name, a string, sans the ".el" extension.
REQUIRES is a list of requirements, each requirement having the
form (NAME VER); NAME is a string and VER is a version list.
DESCRIPTION is the package description, a string.
VERSION is the version, a string.
COMMENTARY is the commentary section, a string, or nil if none.
If the buffer does not contain a conforming package, signal an
error. If there is a package, narrow the buffer to the file's boundaries.