scrapy genspider allows you to create a spider module with the same name as the project

scrapy genspider allows you to create a spider module with the same name as the project leading to import error when you try to use it. This patch checks when creating a spider and prints an error message if the spider name is the same as the project.

```bash
# cd /tmp/
# scrapy startproject testing
# cd testing/
# scrapy genspider testing example.com
Created spider 'testing' using template 'crawl' in module:
  testing.spiders.testing
# scrapy crawl testing
2013-02-12 12:33:17+0000 [scrapy] INFO: Scrapy 0.12.0.2546 started (bot: testing)
2013-02-12 12:33:17+0000 [scrapy] DEBUG: Enabled extensions: TelnetConsole, SpiderContext, WebService, CoreStats, MemoryUsage, CloseSpider
Traceback (most recent call last):
  File "/usr/bin/scrapy", line 4, in <module>
    execute()
  File "/usr/lib/python2.7/dist-packages/scrapy/cmdline.py", line 131, in execute
    _run_print_help(parser, _run_command, cmd, args, opts)
  File "/usr/lib/python2.7/dist-packages/scrapy/cmdline.py", line 97, in _run_print_help
    func(*a, **kw)
  File "/usr/lib/python2.7/dist-packages/scrapy/cmdline.py", line 138, in _run_command
    cmd.run(args, opts)
  File "/usr/lib/python2.7/dist-packages/scrapy/commands/crawl.py", line 42, in run
    q = self.crawler.queue
  File "/usr/lib/python2.7/dist-packages/scrapy/command.py", line 33, in crawler
    self._crawler.configure()
  File "/usr/lib/python2.7/dist-packages/scrapy/crawler.py", line 36, in configure
    self.spiders = spman_cls.from_settings(self.settings)
  File "/usr/lib/python2.7/dist-packages/scrapy/spidermanager.py", line 33, in from_settings
    return cls(settings.getlist('SPIDER_MODULES'))
  File "/usr/lib/python2.7/dist-packages/scrapy/spidermanager.py", line 23, in __init__
    for module in walk_modules(name):
  File "/usr/lib/python2.7/dist-packages/scrapy/utils/misc.py", line 65, in walk_modules
    submod = __import__(fullpath, {}, {}, [''])
  File "/tmp/testing/testing/spiders/testing.py", line 6, in <module>
    from testing.items import TestingItem
ImportError: No module named items
```