internal/extsvc: Use stricter check for equality in CodeHostOf

Created by: indradhanush

In this commit we extract the codehost URL from the given repo name and apply an exact case-insensitive match instead of only checking for the prefix. The previous approach does not handle corner cases like: github.com.example.com/foo/bar and falsely identifies it as a repo belonging to the codehost with URL github.com.