fedora: also check for the spec file in the existence check

Fixes https://pagure.io/fedora-rust/rust2rpm/issue/165.
the following workflow wasn't well supported by the check:

- request compat package repo
- fedpkg clone it
- try to run rust2rpm for it
- Get this message:
  Crate tiny_http0.6 is already packaged in Fedora (...)
  Re-run with --no-existence-check if you still want to convert it.

rust2rpm/__main__.py

@@ -252,11 +252,21 @@ def get_package_info(package):
     url = requests.compat.urljoin(DIST_GIT_URL, f"rpms/{package}")
     req = requests.get(url, headers={"User-Agent": "rust2rpm"})
     json = req.json()
-    if "name" in json:
-        return json
-    else:
+    if 'name' not in json:
         return None
 
+    # E.g. https://src.fedoraproject.org/rpms/rust-tiny_http0.6/blob/rawhide/f/rust-tiny_http0.6.spec
+    full_url = json['full_url']
+    spec_url = requests.compat.urljoin(full_url, f'blob/rawhide/f/rust-{package}.spec')
+    req = requests.head(url, headers={"User-Agent": "rust2rpm"})
+
+    if not req.ok:
+        # The repo exists, but doesn't have the spec file. We most
+        # likely want to create a spec file to add to that repo.
+        return None
+
+    return json
+
 def make_diff_metadata(crate, version, patch=False, store=False):
     if _is_path(crate):
         # Only things that look like a paths are considered local arguments