Merging Finder into package mechanism

[Chong Yidong]

package.el has a variable `package--builtins-base' which defines
packages built into Emacs.  This is similar, but not identical, to the
Finder conception of a package.  I would like to remove the discrepancy.

As part of this, I also want to eliminate some of the useless entries
that currently show up in Finder.  For instance, Finder gives cc-vars.el
a separate entry from cc-mode.el, which is not very useful.  All the
cc-*.el files should get a single entry, under the "cc-mode" package.

Eventually, finder-by-keyword should allow searching for elpa packages
as well as built-in packages.

The proposed technical steps are as follows.  First, merge
`finder-package-info' and `package--builtins-base' into a single
variable.  This is a backward-incompatible change, but I think that's
acceptable, since third-party code is unlikely to make use of
`finder-package-info'.

Second, alter `finder-compile-keywords' to recognize files that are part
of multi-file built-in packages.  I propose to do this by adding an
optional "Package:" file header, which says that the file is part of a
multi-file package.  For instance, cc-vars.el can have the header
"Package: cc-mode".  Files that we want to omit from Finder can have the
header "Package: emacs".

Thoughts?