Remove costly io/resource lookup that wasn't doing anything

The `immutable-source-file?` check is used in `class-info` to know if it makes
sense to invalidate a cache entry based on modification time. This check uses
the `:path` it is given to see if the resource is inside a jar or not.

This `:path` comes from `orchard.java.{parser,legacy-parser}`, and is attained
by calling `(.getPath (io/resource ...))`. This means it is either an absolute
path on the filesystem, or it is a `file:..` URL pointing to a JAR.

```clojure
(io/resource "lambdaisland/witchcraft.clj")
;;=> #java.net.URL "file:/srv/mc/witchcraft/src/lambdaisland/witchcraft.clj"

(.getPath (io/resource "lambdaisland/witchcraft.clj"))
;;=> "/srv/mc/witchcraft/src/lambdaisland/witchcraft.clj"

(io/resource "clojure/lang/RT.class")
;;=> #java.net.URL "jar:file:/root/.m2/repository/org/clojure/clojure/1.10.3/clojure-1.10.3.jar!/clojure/lang/RT.class"

(.getPath (io/resource "clojure/lang/RT.class"))
;;=> "file:/root/.m2/repository/org/clojure/clojure/1.10.3/clojure-1.10.3.jar!/clojure/lang/RT.class"
```

Passing these things to `io/resource` again makes no sense, because these are
absolute paths/urls, they can't be resolved relative to the classpath. This will
always return nil.

```clojure
(io/resource "file:/root/.m2/repository/org/clojure/clojure/1.10.3/clojure-1.10.3.jar!/clojure/lang/RT.class")
;;=> nil
```

This check is performed whether the class-info is cached already or not, causing
a full classpath scan on every call.

The net effect is that depending on circumstances cider-nrepl's "classpath" op
can get extremely slow, around 10~20 seconds to analyze all stack frames and
return a result, which means that the error buffer will only pop up 10~20
seconds after evaluating a form which causes an error.

CHANGELOG.md

@@ -6,6 +6,10 @@
 
 * `orchard.namespace` functionality is now parallelized when possible.
 
+### Bugs Fixed
+
+* [#124](https://github.com/clojure-emacs/orchard/pull/124): Remove costly `io/resource` lookup that wasn't doing anything 
+
 ## 0.7.1 (2021-04-18)
 
 ### Bugs Fixed

src/orchard/java.clj

@@ -255,10 +255,9 @@
   returns true if no source file is available, or if the source file is in a
   jar/zip archive."
   [info]
-  (let [path (:file info)
-        src  (when path (io/resource path))]
-    (or (not src)
-        (re-find #"\.(jar|zip)!" (str src)))))
+  (let [path (:file info)]
+    (or (not path)
+        (re-find #"\.(jar|zip)!" (str path)))))
 
 (defn class-info
   "For the class symbol, return (possibly cached) Java class and member info.