wip: make gen_implib a rule

Note: This does **not** resolve the rules_python host error when
building wheels on Linux, but seems like a good starting point? And,
improves readability + consistency with other Drake build rules w.r.t.
genrule's makefile semantics.

Author: tyler-yankee

tools/workspace/mosek/gen_implib.bzl

@@ -0,0 +1,40 @@
+_PY_CC_TOOLCHAIN_TYPE = "@rules_python//python/cc:toolchain_type"
+
+def _impl(ctx):
+    args = ctx.actions.args()
+    args.add("--dlopen-callback", ctx.attr.dlopen_callback)
+    args.add("--outdir", ctx.outputs.outs[0].dirname)
+    args.add(ctx.file.src.path)
+
+    py_toolchain = ctx.toolchains[_PY_CC_TOOLCHAIN_TYPE]
+
+    ctx.actions.run(
+        mnemonic = "GenImplib",
+        executable = ctx.executable.tool,
+        arguments = [args],
+        inputs = [ctx.file.src],
+        outputs = ctx.outputs.outs,
+        tools = [
+            ctx.executable.tool,
+            py_toolchain.py3_runtime.files,
+        ]
+    )
+
+    return [DefaultInfo(
+        files = depset(ctx.outputs.outs),
+    )]
+
+gen_implib = rule(
+    implementation = _impl,
+    attrs = {
+        "src": attr.label(allow_single_file = True, mandatory = True),
+        "tool": attr.label(
+            mandatory = True,
+            executable = True,
+            cfg = "target",
+        ),
+        "dlopen_callback": attr.string(mandatory = True),
+        "outs": attr.output_list(mandatory = True),
+    },
+    toolchains = ["@rules_python//python/cc:toolchain_type"]
+)

tools/workspace/mosek/package.BUILD.bazel

@@ -4,6 +4,7 @@ load("@bazel_skylib//lib:paths.bzl", "paths")
 load("@bazel_skylib//rules:common_settings.bzl", "bool_flag")
 load("@drake//tools/install:install.bzl", "install", "install_files")
 load("@drake//tools/skylark:cc.bzl", "cc_import", "cc_library")
+load("//tools/workspace/mosek:gen_implib.bzl", "gen_implib")
 
 package(default_visibility = ["//visibility:private"])
 
@@ -79,12 +80,12 @@ _IMPLIB_SRCS = [
     "{}.init.c".format(_MOSEK_SO_NAME),
 ]
 
-genrule(
+gen_implib(
     name = "_gen_implib",
-    srcs = [_MOSEK_MAJMIN_GLOB[0]],
+    src = _MOSEK_MAJMIN_GLOB[0],
     outs = _IMPLIB_SRCS,
-    cmd = "$(execpath @implib_so_internal//:gen) --dlopen-callback=drake_dlopen_mosek --outdir=$(RULEDIR) $<",
-    tools = ["@implib_so_internal//:gen"],
+    drake_dlopen_mosek = "drake_dlopen_mosek",
+    tool = "@implib_so_internal//:gen",
 )
 
 cc_library(