Replace url_override with base_url (#43)

In practice, this is the only part we need to replace, the actual file
name stays the same. It also helps with simplifying the wheel unpacking
logic in modular/modular, as we can reuse this value in more places, and
override it all in one place.

Author: Ahajha

mojo/extensions.bzl

@@ -29,8 +29,10 @@ _NULL_SHAS = {
 }
 
 def _mojo_toolchain_impl(rctx):
+    base_url = rctx.attr.base_url or "https://dl.modular.com/public/nightly/python"
     rctx.download_and_extract(
-        url = rctx.attr.url_override or "https://dl.modular.com/public/nightly/python/max-{}-py3-none-{}.whl".format(
+        url = "{}/max-{}-py3-none-{}.whl".format(
+            base_url,
             rctx.attr.version,
             _PLATFORM_MAPPINGS[rctx.attr.platform],
         ),
@@ -60,8 +62,8 @@ _mojo_toolchain_repository = repository_rule(
             values = _PLATFORMS,
             mandatory = True,
         ),
-        "url_override": attr.string(
-            doc = "Override the download URL for the prebuilt package.",
+        "base_url": attr.string(
+            doc = "Override the base download URL for the prebuilt package.",
             default = "",
         ),
         "use_prebuilt_packages": attr.bool(
@@ -115,7 +117,7 @@ def _setup_toolchains(root_module, rules_mojo_module):
             name = name,
             version = tags.version,
             platform = platform,
-            url_override = tags.url_override,
+            base_url = tags.base_url,
             use_prebuilt_packages = tags.use_prebuilt_packages,
         )
 
@@ -165,8 +167,8 @@ _toolchain_tag = tag_class(
             doc = "The version of the Mojo toolchain to download.",
             default = _DEFAULT_VERSION,
         ),
-        "url_override": attr.string(
-            doc = "Override the download URL for the prebuilt package.",
+        "base_url": attr.string(
+            doc = "Override the base download URL for the prebuilt package.",
             default = "",
         ),
         "use_prebuilt_packages": attr.bool(