Fix snapshot version support for leiningen

[JohnnyJayJay]

It's me again, fixing Leiningen stuff as I encounter it.

The current code doesn't support the use of snapshot version from Leiningen, due to the caching being implemented via missing-mvn-deps. Here's the error you encounter if you try, for reference:

Error

{:clojure.main/message
 "Execution error (AssertionError) at cljnix.utils/mvn-repo-info (utils.clj:134).\nAssert failed: (if (snapshot? path) (not (nil? exact-version)) true)\n",
 :clojure.main/triage
 {:clojure.error/class java.lang.AssertionError,
  :clojure.error/line 134,
  :clojure.error/cause
  "Assert failed: (if (snapshot? path) (not (nil? exact-version)) true)",
  :clojure.error/symbol cljnix.utils/mvn-repo-info,
  :clojure.error/source "utils.clj",
  :clojure.error/phase :execution},
 :clojure.main/trace
 {:via
  [{:type java.lang.AssertionError,
    :message
    "Assert failed: (if (snapshot? path) (not (nil? exact-version)) true)",
    :at [cljnix.utils$mvn_repo_info invokeStatic "utils.clj" 134]}],
  :trace
  [[cljnix.utils$mvn_repo_info invokeStatic "utils.clj" 134]
   [cljnix.utils$mvn_repo_info doInvoke "utils.clj" 134]
   [clojure.lang.RestFn invoke "RestFn.java" 423]
   [cljnix.core$missing_mvn_deps$fn__22429 invoke "core.clj" 176]
   [clojure.core$map$fn__5931$fn__5932 invoke "core.clj" 2759]
   [clojure.core$filter$fn__5958$fn__5959 invoke "core.clj" 2823]
   [clojure.core$map$fn__5931$fn__5932 invoke "core.clj" 2759]
   [clojure.core$map$fn__5931$fn__5932 invoke "core.clj" 2759]
   [clojure.core$filter$fn__5958$fn__5959 invoke "core.clj" 2823]
   [clojure.lang.PersistentVector reduce "PersistentVector.java" 343]
   [clojure.core$transduce invokeStatic "core.clj" 6947]
   [clojure.core$into invokeStatic "core.clj" 6963]
   [clojure.core$into invoke "core.clj" 6951]
   [cljnix.core$missing_mvn_deps invokeStatic "core.clj" 168]
   [cljnix.core$missing_mvn_deps invoke "core.clj" 160]
   [cljnix.core$lock_file$fn__22521 invoke "core.clj" 398]
   [clojure.core$completing$fn__8528 invoke "core.clj" 6931]
   [clojure.core$map$fn__5931$fn__5932 invoke "core.clj" 2757]
   [clojure.core$cat$fn__8851 invoke "core.clj" 7696]
   [clojure.core$map$fn__5931$fn__5932 invoke "core.clj" 2757]
   [clojure.core$map$fn__5931$fn__5932 invoke "core.clj" 2757]
   [clojure.core$transduce invokeStatic "core.clj" 6949]
   [clojure.core$transduce invoke "core.clj" 6934]
   [cljnix.core$lock_file invokeStatic "core.clj" 381]
   [cljnix.core$lock_file invoke "core.clj" 372]
   [cljnix.core$_main invokeStatic "core.clj" 508]
   [cljnix.core$_main doInvoke "core.clj" 503]
   [clojure.lang.RestFn applyTo "RestFn.java" 137]
   [clojure.lang.Var applyTo "Var.java" 705]
   [clojure.core$apply invokeStatic "core.clj" 667]
   [clojure.main$main_opt invokeStatic "main.clj" 514]
   [clojure.main$main_opt invoke "main.clj" 510]
   [clojure.main$main invokeStatic "main.clj" 664]
   [clojure.main$main doInvoke "main.clj" 616]
   [clojure.lang.RestFn applyTo "RestFn.java" 137]
   [clojure.lang.Var applyTo "Var.java" 705]
   [clojure.main main "main.java" 40]],
  :cause
  "Assert failed: (if (snapshot? path) (not (nil? exact-version)) true)"}}

I've spent some time debugging and trying to understand the logic of mvn-repo-info, and while I'm not entirely sure I get it, I found that simply setting the :exact-version of any "missing dep" to "SNAPSHOT" would circumvent the issue because it results in the desired behaviour.

I realise that this might be a hack. Let me know if there is a flaw in what I've done here or if you see other ways to mitigate the issue.

[jlesquembre]

Reviewing that logic again, I guess the :pre check in mvn-repo-info was added only for debugging purposes and was committed by mistake. We can remove it, which should fix the issue in Leiningen.

Maybe we can remove the need for the exact-version argument, but I prefer to wait to do that.