feat: expand the usefulness of resource_sets (#1235)

This adds a new public function, `resource_set_for`, which takes a cpu
and memory value and returns an appropriate `resource_set` function for
those values, adding some support for the cross-product [mentioned in
the previous
pass](#792 (comment)).
The bazel resource_set API isn't very easy to work with, and this hides
the ugliness here, rather than expecting every module to implement
something like [rules_rust
did](https://github.com/bazelbuild/rules_rust/blob/f29a63cb3c473bd0158c8c9d3e0793a33187d505/rust/private/rustc_resource_set.bzl).

The ugliness of the implementation is mitigated slightly by using a
helper script to generate it, and I reason that people attempting to use
the `resource_set` API would be better off with additional abstraction,
so if/when this gets fixed in bazel, most people don't need to notice.

Author: novas0x2a

lib/BUILD.bazel

@@ -276,4 +276,5 @@ bzl_library(
 bzl_library(
     name = "resource_sets",
     srcs = ["resource_sets.bzl"],
+    deps = ["//lib/private:resource_sets"],
 )

lib/private/BUILD.bazel

@@ -289,6 +289,12 @@ bzl_library(
     visibility = ["//lib:__subpackages__"],
 )
 
+bzl_library(
+    name = "resource_sets",
+    srcs = ["resource_sets.bzl"],
+    visibility = ["//lib:__subpackages__"],
+)
+
 bzl_library(
     name = "source_toolchains_repo",
     srcs = ["source_toolchains_repo.bzl"],