Implement Ultra-Lazy-Load

[naftulikay]

Presently with #16, we support lazy-loading of credential files when a qualified profile URI is used. If you have a profile named dev in ~/.aws/credentials.d/92-something.gpg, the profile URI will be dev/92-something; if stg exists in ~/.aws/credentials, then the profile URI will be /stg.

Currently, lazy-loading works for root URIs, but if a non-root profile URI is used, we load all profiles rather than incrementally try to load by the file stem. The most optimal solution would be to lazy load like this:

• ✔️ If passed a root URI:
  1. Load the root credentials file and return profile if found.
  2. Load all credentials and return profile if found.
• 🚫 If passed a non-root URI:
  1. Load files matching the profile URI prefix as a file stem, and return profile if found.
  2. Load all credentials and return profile if found.
• ✔️ If passed a profile name and not a URI, load all files and return profile if found.

Implementing the non-root URI lazy-loading is what is required by this issue.