The CLI used to support this:

```
[profile sso]
...

[profile one]
...
source_profile = sso
```

But not this:

```
[profile sso]
...

[profile one]
...
source_profile = sso

[profile two]
...
source_profile = one
```

The reason was that:

* We have to do an explicit detection of SSO source profiles in our
  `PatchedSharedIniFileCredentials` because the upstream
  `SharedIniFileCredentials` has no SSO support at all; and
* When we recursed we would recurse using the `SharedIniFileCredentials`
  class.

In combination, this means that we could only recurse **one level**,
because in `SharedIniFileCredentials` we wouldn't support SSO profiles
at all.

Fix this by recursing using `SharedIniFileCredentials`, so that we can
support SSO source profiles an arbitrary amount of nesting levels deep.

While investigating this, also fixed the following issues:

- SSO profiles would be detected using the incorrect key:
  `sso_start_url` can be specified either on the profile section,
  or a new `[sso-session]` section. `sso_account_id` however always
  must be on the profile section, so check on that.
- Drop the `credentials/config` file loading patch. The upstream SDK
  has fixed this behavior in the mean time, so we can rely on the
  passed-in profile data again.
- Dropped support for reading the STS AssumeRole `region` from the
  `[default]` section. After investigating by both the JS SDK team
  and myself, noticed that the AWS CLI does **not** support reading
  the region from there. While we are both in agreement this is
  a bug, all customers expect the CDK CLI to behave exactly the same
  as the AWS CLI, so we have to keep bug-for-bug compatibility.