fix(manifest): Fix bugs in 'Get-Manifest()'

[niheaven]

Description

There're some bugs in #4966 and one of them is fixed in #4970. This PR fixes them all.

• 'Get-Dependency()' may return wrong app name when it's a path
• Trim starting "/" if there is no bucket
• Don't return wrong 'versions' bucket for local manifest

Motivation and Context

• Closes scoop install mupdf@<version> fails #4984
• Related refactor(manifest): Rename 'Find-Manifest()' to 'Get-Manifest()' #4966
• Related fix(scoop-download): Use correct Args when calling Get-Manifest #4970

How Has This Been Tested?

• scoop install xxx@xxx

• scoop install \path\to\json\that\not\existed

• scoop install \path\to\json

Checklist:

• I have read the Contributing Guide.
• I have ensured that I am targeting the develop branch.
• I have updated the documentation accordingly.
• I have updated the tests accordingly.
• I have added an entry in the CHANGELOG.