fixed svn packages installation with explicit revisions

fixed a bug with svn-package requirements on an exact commit.

before it trys to download something like that:

*package exists as branch 0.3 in revision 4711*

`https://svn.example.com/package/4711` thats obviously wrong...

now it downloads the correct path:

`https://svn.example.com/package/branches/0.3@4711`