Vendor dependencies

Let's see how I like this workflow.

vendor/cxx/tools/buck/prelude/remote_file.bzl

@@ -0,0 +1,50 @@
+# Copyright (c) Meta Platforms, Inc. and affiliates.
+#
+# This source code is licensed under both the MIT license found in the
+# LICENSE-MIT file in the root directory of this source tree and the Apache
+# License, Version 2.0 found in the LICENSE-APACHE file in the root directory
+# of this source tree.
+
+load("@prelude//:http_file.bzl", "http_file_shared")
+load("@prelude//utils:utils.bzl", "expect", "value_or")
+
+_ROOT = "https://maven.thefacebook.com/nexus/content/groups/public"
+
+def _from_mvn_url(url: str.type) -> str.type:
+    """
+    Convert `mvn:` style URIs to a URL.
+    """
+
+    mvn, group, id, typ, version = url.split(":")
+    expect(mvn == "mvn")
+
+    group = group.replace(".", "/")
+
+    if typ == "src":
+        ext = "-sources.jar"
+    else:
+        ext = "." + typ
+
+    return "{root}/{group}/{id}/{version}/{id}-{version}{ext}".format(
+        root = _ROOT,
+        group = group,
+        id = id,
+        version = version,
+        ext = ext,
+    )
+
+# Implementation of the `remote_file` build rule.
+def remote_file_impl(ctx: "context") -> ["provider"]:
+    url = ctx.attrs.url
+    if url.startswith("mvn:"):
+        url = _from_mvn_url(url)
+    return http_file_shared(
+        ctx.actions,
+        name = value_or(ctx.attrs.out, ctx.label.name),
+        url = url,
+        is_executable = ctx.attrs.type == "executable",
+        is_exploded_zip = ctx.attrs.type == "exploded_zip",
+        unzip_tool = ctx.attrs._unzip_tool[RunInfo],
+        sha1 = ctx.attrs.sha1,
+        sha256 = ctx.attrs.sha256,
+    )