Change visibility of svm-internal structs and APIs

[pgarg66]

Problem

Some APIs and structs in SVM crates are only used internally (among these crates). The visibility should be limited to these crates.

Summary of Changes

Limit the visibility using svm-internal cfg feature.

Fixes #

[mergify]

The Firedancer team maintains a line-for-line reimplementation of the
native programs, and until native programs are moved to BPF, those
implementations must exactly match their Agave counterparts.
If this PR represents a change to a native program implementation (not
tests), please include a reviewer from the Firedancer team. And please
keep refactors to a minimum.

[apfitzge]

I'm confused by these qualifiers. if they are not pub now why do we need them to be?

[pgarg66]

Removing the qualifiers here causes the following build error:

warning: type `BuiltinCost` is more private than the item `MIGRATING_BUILTINS_COSTS`
   --> builtins-default-costs/src/lib.rs:120:49
    |
120 |   #[cfg_attr(feature = "svm-internal", qualifiers(pub))]
    |  _________________________________________________^
121 | | const MIGRATING_BUILTINS_COSTS: &[(Pubkey, BuiltinCost)] = &[
    | |________________________________________________________^ constant `MIGRATING_BUILTINS_COSTS` is reachable at visibility `pub`
    |
note: but type `BuiltinCost` is only usable at visibility `pub(crate)`
   --> builtins-default-costs/src/lib.rs:42:1
    |
42  | enum BuiltinCost {
    | ^^^^^^^^^^^^^^^^
    = note: `#[warn(private_interfaces)]` on by default

   Compiling solana-compute-budget-instruction v2.2.0 (/Users/pankaj/workspace/solana/compute-budget-instruction)
warning: `solana-builtins-default-costs` (lib) generated 1 warning
error: type `solana_builtins_default_costs::BuiltinCost` is private
  --> compute-budget-instruction/src/compute_budget_instruction_details.rs:31:48
   |
31 |             migrating_builtin: [Saturating(0); MIGRATING_BUILTINS_COSTS.len()],
   |                                                ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ private type

error: could not compile `solana-compute-budget-instruction` (lib) due to 1 previous error
warning: build failed, waiting for other jobs to finish...

And, MIGRATING_BUILTINS_COSTS is used by compute-budget-instruction/src/compute_budget_instruction_details.rs. So it needs to be public (at least within SVM internal crates).

It's a bit convoluted. I had to go through multiple compilation failures to get to this set of qualifiers. I wish it could be cleaner.