Support root level nested value lookups

[lukasbindreiter]

What type of PR is this?

• bug

What this PR does / why we need it:

It fixes a bug in NestedVal where lookup doesn't work for keys directly in the first level (root) of the map.

Which issue(s) this PR fixes:

Fixes #14

Special notes for your reviewer:

The issue is related to this one I raised in cli: urfave/cli#2037
I created a related PR there as well: urfave/cli#2047

The source code of NestedVal, and even the tests seem to be a copy paste from urfave/cli in this case.
I kept it now like this as well - though there would be a possibility to remove that duplication by moving the NestedVal function to urfave/cli directly, make it public and use it there inside MapSource.Lookup and then also in cli-altsrc.

If you prefer that approach I could update my PRs accordingly - though this would require the urfave/cli one to be merged and released before this PR can be updated to rely on it.

Testing

I added a test case for this exact issue as well. It will fail without changes from the PR.
I also added another test case for the map[string]any special case that was in place in the code, to make sure that works to.

[dearchap]

@lukasbindreiter Amazing work. Thanks for the PR. !!!

[lukasbindreiter]

@dearchap CI was failing due to some unrelated test case - connecting to machines on a local network seems to be disabled on Github CI - I attempted to fix it by changing the expected error to a substring that covers both cases.