* gnu/packages/emacs-xyz.scm (emacs-telega): Update to 0.6.27-1.

[source]: Take a new commit that includes the latest .el files from the
contrib subdirectory.

[arguments]: Add a substitute method for the 'telega-install-contrib phase to
know where telega-contrib is located, and use it appropriately.

[arguments]: Manually invoke bytecompilation as a separate step for the
contrib directory.

[arguments]: Add 'telega-install-contrib phase to place the contrib directory
files in a more guix-suitable place.