NEXUS-5838: RRB uses wrong path in case of some index pages

Code was prepared for two possibilities: that HTML `a` tag's `href`
attribute contains relative paths (taken from index page base),
or, that `href` attribute contains full URLs.

But here we have a third (valid) case: the `href` attribute contains
absolute paths from the server root.

Hence, the paths were wrongly assembled, as the basePath
element was duplicated in the URL requested from remote.

Issue
https://issues.sonatype.org/browse/NEXUS-5838

CI
https://bamboo.zion.sonatype.com/browse/NX-OSSF36