CRM-19826 - CRM_Extension_System - Fix extra slash in `vendor` URLs

CRM-19826 - CRM_Extension_System - Fix extra slash in `vendor` URLs

If you install an extension under `$cmsRoot/vendor/org.example.foo`, the
resulting URL contains an extraneous `/` (eg
`http://example.org//vendor/org.example.foo`).  (In Windows, I suspect it's
even worse because it uses DIRECTORY_SEPARATOR in the URl -- eg
`http://example.org/\vendor/org.example.foo`.)

This patch checks for and removes the extraneous slash -- and always
constructs the URL with the appropriate delimiter (`/`).

Problem observed in `dmaster`.