aya: tidy bpf syscalls and feature support functions into submodules

This organizes the bpf syscalls and feature probe functions in
aya/src/sys/bpf.rs into submodules: btf, link, map, object, program,
utils, and feature_probe. This is to help make development slightly
less daunting to parse and expand on.

The submodules, except for feature_probe, are re-exported so that
importing them remains unchanged (i.e. crate::sys::foo). The feature
probe submodule isn't re-exported (uses crate::sys::feature_probe::foo)
so that there's some separation between bpf syscall stuff and feature
probing.

aya/src/bpf.rs

@@ -36,11 +36,14 @@ use crate::{
         SkMsg, SkSkb, SkSkbKind, SockOps, SocketFilter, TracePoint, UProbe, Xdp,
     },
     sys::{
-        bpf_load_btf, is_bpf_cookie_supported, is_bpf_global_data_supported,
-        is_btf_datasec_supported, is_btf_decl_tag_supported, is_btf_enum64_supported,
-        is_btf_float_supported, is_btf_func_global_supported, is_btf_func_supported,
-        is_btf_supported, is_btf_type_tag_supported, is_perf_link_supported,
-        is_probe_read_kernel_supported, is_prog_id_supported, is_prog_name_supported,
+        bpf_load_btf,
+        feature_probe::{
+            is_bpf_cookie_supported, is_bpf_global_data_supported, is_btf_datasec_supported,
+            is_btf_decl_tag_supported, is_btf_enum64_supported, is_btf_float_supported,
+            is_btf_func_global_supported, is_btf_func_supported, is_btf_supported,
+            is_btf_type_tag_supported, is_perf_link_supported, is_probe_read_kernel_supported,
+            is_prog_id_supported, is_prog_name_supported,
+        },
         retry_with_verifier_logs,
     },
     util::{bytes_of, bytes_of_slice, page_size, possible_cpus, POSSIBLE_CPUS},