Fix configs which use a root path, and subpaths

[dnephin]

Trimming a / from the path / resulted in a broken config.

Example:

paths:
  /:
    repo: repo1

  /middle:
    repo: repo2

Using this config any subpackage in repo1 that sorted before middle would work, but anything after would cause a 404 response. This can be reproduced in TestPathConfigSetFind by changing any paths of "/" in the paths field to "" (which is actually what find() was receiving).

With this patch, / urls are preserved as / and all subpackages of repo1 will be available from the vanity url, so TestPathConfigSetFind accurately reflects the behaviour of find().