bug#20770: 24.5; feature request: support relative path in package.el

[Oleh Krehel]

Suppose I my config directory looks like this:

    elpa/
    etc/
    Makefile
    init.el
    packages.el

The ELPA packages go into elpa/, and packages.el looks like:

    (dolist (package '("slime"))
      (unless (package-installed-p package)
        (package-install package)))

And I install/upgrade my packages with a Makefile:

    emacs -batch -l packages.el 2>&1 | tee etc/log

But now, if I open etc/log, all byte compilation warnings generated by
package.el are relative to elpa/slime-20150601.1411/, so I can't jump to
them if I enable `compilation-mode'.

Obviously, this isn't high priority, but it would be cool if we could
have:

    (defvar package-target-path "."
      "The byte-compilation warnings will target this path.")

So I could set it temporarily to "~/.emacs.d/log/", and then instead of
this:

    In slime-parse-extended-operator-name:
    contrib/slime-enclosing-context.el:45:36:Warning: reference to free
    variable `slime-extended-operator-name-parser-alist'

I could get this:

    In slime-parse-extended-operator-name:
    
../elpa/slime-20150601.1411/contrib/slime-enclosing-context.el:45:36:Warning:
    reference to free variable `slime-extended-operator-name-parser-alist'