fix: fallback to updatecli.yaml or updatecli.d if no manifest specified

[olblak]

Fix #1694

Test

To test this pull request, you can run the following commands:

I build the binary running go build -o bin/updatecli . then I executed from a directory:

• with a file named updatecli.yaml then it tried to load it
• with a directory named updatecli.d then it tried to load manifests from it
• with both a file named updatecli.yaml and updatecli.d then it tried to load both manifests
• without a file named updatecli.yaml or updatecli.d then it tried to load autodiscovery

Additional Information

Tradeoff

Potential improvement

[mcwarman]

Tested the 4 scenarios working as expected.

Also retested project where found #1694 and working as expected.

[olblak]

I reorganize a bit the code as I find it clearer now.
After additional thought, the constant should never have been in a different package, I lost track of that.

[mcwarman]

The fact it was there and not reachable makes more sense than it getting removed.

I found the constants when I was first looking into the bug.