add packages to find-function-source-path when set

When find-function-source-path is nil, emacs searches load-path
instead, so e.g. find-library works fine with el-get.  However, when
find-function-source-path is non-nil, find-library will search that
instead, in which case el-get needs to add packages to it.

Fixes #1167.

https://github.com/dimitri/el-get/issues/1167