Change repository directory layout

* Under the base URL there are supposed to be the repository files and a
  subdirectory "packages".
* Fix the repository URL related confusion introduced earlier. The URL
in
  the repository info (and thus in the repository file) is supposed to
  be the base URL for the repository. It is not a (potentially)
  different base URL for the package files. Package and repository
  files were supposed to live in the same directory. Now, by requiring
  the package files to live in a subdirectory -- which can also be a
  symlink -- we gain some flexibility.
  The URL in the repository config is usually the same as the in the
  repository info, unless it refers to a mirror site. This allows for
  mirrors to copy the original repository verbatim.
* Remove the PackageURL rule and introduce a DownloadPackage rule
  instead. The URL for a package file cannot be computed in the jam
  parsing phase anymore, as it contains the hash value of the package
  list.
* BRepositoryConfig: Add PackagesURL() for convenience.

headers/os/package/RepositoryConfig.h

@@ -33,6 +33,8 @@ public:
 
 			const BEntry&		Entry() const;
 
+			BString				PackagesURL() const;
+
 			void				SetName(const BString& name);
 			void				SetBaseURL(const BString& url);
 			void				SetPriority(uint8 priority);