Fix CI step for `cargo semver checks` (#4033)

## Motivation and Context
Fixes CI step for running `cargo semver checks`

## Description
The said CI step has been failing for the past three months. The most
recent failure observed in CI looks like the following, since we
[upgraded the version of `cargo-semver-checks` to 0.36
](https://github.com/smithy-lang/smithy-rs/pull/3936)
```
error: package `aws-runtime` is ambiguous: it is defined by in multiple manifests within the root path /home/build/workspace/smithy-rs/target/semver-checks/git-base/be980cd7049d9acdb38f68dd11c465b3a242733a

defined in:
  /home/build/workspace/smithy-rs/target/semver-checks/git-base/be980cd7049d9acdb38f68dd11c465b3a242733a/aws/rust-runtime/aws-runtime/Cargo.toml
  /home/build/workspace/smithy-rs/target/semver-checks/git-base/be980cd7049d9acdb38f68dd11c465b3a242733a/tmp-codegen-diff/aws-sdk/sdk/aws-runtime/Cargo.toml
```
Referring to the diagram in [a previous relevant
PR](https://github.com/smithy-lang/smithy-rs/pull/3272), we see that the
`tmp-codegen-diff` directory is used as [the output
directory](https://github.com/smithy-lang/smithy-rs/blob/e394ad8b099ea638e0f9c7549295aaebccc36a61/tools/ci-scripts/codegen-diff/diff_lib.py#L11)
by `diff_lib.py`. However, the current version of `cargo semver checks`
no longer supports duplicated crates within the same root directory
([PR](https://github.com/obi1kenobi/cargo-semver-checks/pull/887)).

This requires a solution where we maintain a single source of crate
layout to validate against `cargo-semver-checks` under `smithy-rs`. To
address this, this PR implements the following approach:
- (Bonus point) Reverts the changes made in [the previous relevant
PR](https://github.com/smithy-lang/smithy-rs/pull/3272).
- Updates `semver-checks.py` to stop using
`checkout_commit_and_generate` in `diff_lib.py`, which automatically
places both the runtime crates and the generated SDK crates into
`tmp-codegen-diff`, leading to conflicts with the crates in
`rust-runtime` and in `aws/rust-runtime`.
- When `semver-checks.py` creates the `base` and `current` branches, we
remove runtime crates from both `rust-runtime` and `aws/rust-runtime` to
uniquify the runtime crates in the `aws/sdk/build/aws-sdk/sdk`
directory. Note that the removal of crates only occurs in the `base` and
`current` branches and does not impact the main branch.
- Moves the `aws/sdk/build/aws-sdk` directory to the root of `smithy-rs`
for a reason explained in dd021d1.

## Testing
- CI passed `cargo semver checks`
- Removed [this enum
variant](https://github.com/smithy-lang/smithy-rs/blob/e394ad8b099ea638e0f9c7549295aaebccc36a61/rust-runtime/aws-smithy-runtime/src/client/sdk_feature.rs#L13)
intentionally to cause an API breakage, the step correctly detected it
```
--- failure enum_variant_missing: pub enum variant removed or renamed ---

Description:
A publicly-visible enum has at least one variant that is no longer available under its prior name. It may have been renamed or removed entirely.
        ref: https://doc.rust-lang.org/cargo/reference/semver.html#item-remove
       impl: https://github.com/obi1kenobi/cargo-semver-checks/tree/v0.36.0/src/lints/enum_variant_missing.ron

Failed in:
  variant BusinessMetric::GzipRequestCompression, previously in file /Users/awsaito/src/smithy-rs/target/semver-checks/git-base/d87d9ee34a13da0ad4c0cdcbfa2c74b7f98d278c/aws-sdk/sdk/aws-runtime/src/user_agent/metrics.rs:108
```

----

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