Simplify `trac.loader` a bit by:  * making use of the fact that SetupTools is now a mandatory dependency, and  * dropping support for plugins not using the `trac.plugins` entry point. Also, load `.py` source (single-file) plugins after Python eggs (closes #4383).
Simplify `trac.loader` a bit by:
 * making use of the fact that SetupTools is now a mandatory dependency, and
 * dropping support for plugins not using the `trac.plugins` entry point.
Also, load `.py` source (single-file) plugins after Python eggs (closes #4383).

git-svn-id: eda3d06fcef731589ace1b284159cead3416df9b@4633 af82e41b-90c4-0310-8c96-b1721e28e2e2

jun66j5/trac-ja,netjunki/trac-Pygit2,netjunki/trac-Pygit2,walty8/trac,walty8/trac,jun66j5/trac-ja,jun66j5/trac-ja,jun66j5/trac-ja,netjunki/trac-Pygit2,walty8/trac,walty8/trac