hugolib: Normalize permalink path segments

hugolib: Normalize permalink path segments

When constructing permalinks, ensure that most inputs used as path
segments are normalized with PathSpec.MakeSegment instead of
PathSpec.URLize.

The primary exception to that rule is with taxonomy titles in
pageToPermalinkTitle(). The approach taken here is to use URLize for
taxonomy pages. Everything else will use MakeSegment. The reason for
this exception is that people use taxonomies such as "s1/p1" to generate
URLs precisely they way they wish (see #5223). Tests have been added to
check for this case.

Fixes #4926