Add emacs-source-reference for C code discoverablity.

[Zhu Zihao]

GNU Emacs can navigate and reference the source code of C function. But
the variable source-directory should be setup properly.

Current Emacs package discard C source code during the build. There's
serveral solution to add it.

1. Modify the emacs package definition, this trigger the rebuild of
all emacs-xyz, and increase the closure for who doesn't want the source
reference.

2. Like 1. But put the source to another output in emacs package. Still
trigger rebuild but doesn't increase closure size. 

3. Create a new package named emacs-source-reference to hold it. I think
this is the better way.

BTW, if we add source reference, what about also generate etags file for
them?

Please leave your comments on these ideas. Thanks a lot.
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Zihao

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