There are several issues with this method, such as #141, #112 probably
some others, which says there are problem installing plugins.
The problme is that `sourced_files` method doesn't always correctly
identifies sources files, so plugin settings defined in those
configuration files would not be loaded.
This change fixes this by doing next things:
* excluding `tmux source` command, which could present in config
* greping all `source` lines
* awk command is left original
* using `eval`, so that globed files `source *.conf` would be loaded
as well
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