When we want to handle a local path, we must rewrite it internally into
a `file:///` URL, because we dispatch our standalone transfer agent
based on the URL scheme. However, once we've looked up an appropriate
URL and formatted it as the scheme prefers, if it contains a trailing
slash, we ignore the entire lookup and replace the URL with the raw one
given, but without the trailing slash.
This behaviour seems a bit bizarre, and it causes us to take a local
path with a trailing slash and treat it not as the `file:///` URL it
needs to be, but as a raw local path, which gets refused. Let's avoid
this problem by looking at the rewritten URL, and modifying that instead
if we find its trailing slash to be offensive.
600132c4f2