fetchTree: Allow fetching plain files

Add a new `file` fetcher type, which will fetch a plain file over
http(s), or from the local file.

Because plain `http(s)://` or `file://` urls can already correspond to
`tarball` inputs (if the path ends-up with a know archive extension),
the URL parsing logic is a bit convuluted in that:

- {http,https,file}:// urls will be interpreted as either a tarball or a
  file input, depending on the extensions of the path part (so
  `https://foo.com/bar` will be a `file` input and
  `https://foo.com/bar.tar.gz` as a `tarball` input)
- `file+{something}://` urls will be interpreted as `file` urls (with
  the `file+` part removed)
- `tarball+{something}://` urls will be interpreted as `tarball` urls (with
  the `tarball+` part removed)

Fix #3785

Co-Authored-By: Tony Olagbaiye <me@fron.io>

doc/manual/src/release-notes/rl-next.md

@@ -24,3 +24,6 @@
 
   Selecting derivation outputs using the attribute selection syntax
   (e.g. `nixpkgs#glibc.dev`) no longer works.
+
+* `builtins.fetchTree` (and flake inputs) can now be used to fetch plain files
+  over the `http(s)` and `file` protocols in addition to directory tarballs.