opam: Initialize opam root directory outside the constructor

If it's required at all, this will use the network to fetch and install it. This should be done outside the constructor.

Related to T3590

Test Plan

tox

---

Migrated from D6317 (view on Phabricator)

swh/loader/package/opam/loader.py

@@ -84,8 +84,13 @@ class OpamLoader(PackageLoader[OpamPackageInfo]):
         self.opam_url = opam_url
         self.opam_package = opam_package
 
-        if not os.path.isdir(opam_root):
-            if os.path.isfile(opam_root):
+    def get_versions(self) -> List[str]:
+        """First initialize the opam root directory if needed the start listing the package
+versions.
+
+        """
+        if not os.path.isdir(self.opam_root):
+            if os.path.isfile(self.opam_root):
                 raise ValueError("invalid opam root")
             else:
                 call(
@@ -96,15 +101,14 @@ class OpamLoader(PackageLoader[OpamPackageInfo]):
                         "--bare",
                         "--no-setup",
                         "--root",
-                        opam_root,
-                        opam_instance,
-                        opam_url,
+                        self.opam_root,
+                        self.opam_instance,
+                        self.opam_url,
                     ]
                 )
-        elif not os.path.isfile(os.path.join(opam_root, "config")):
+        elif not os.path.isfile(os.path.join(self.opam_root, "config")):
             raise ValueError("invalid opam root")
 
-    def get_versions(self) -> List[str]:
         versions = opam_read(
             [
                 "opam",