pack: the case warning says only what happens to names that differ in case

[enesismail]

What was wrong. The warning for names that differ only in capitalisation said they "would collide once the site is served" and that the site "will serve whichever the platform kept". Neither is true. Every step that stores and serves a site keeps a path's letter case: each spelling is its own file at its own address, and an address with the wrong case is a page that is not there. That was checked on a live site: the real address answered, and the same address with one letter's case changed did not.

What changes.

• The message is now "N file names differ only in capitalisation:", followed by the names as before.
• The next step reads: "If they end up on the site, each is its own address, and a link has to match its capitalisation exactly. If they are meant to be one file, keep one; otherwise rename one so the names differ by more than case."
• It is conditional because the detector runs over the whole walked tree, and a pair outside the published folders (two components under the source folder, say) never reaches the site. The detector, its id, its severity and its grouping are unchanged.
• The function's comment says the same, and no longer says the names "would become one file".

Row (written first, and red against the old text on every assertion): the exact message and next step for a two-name and a three-name group, read from the function's own output, plus a refusal of "collid" and "whichever the platform kept" in either field, case-folded.

Mutations, each run and reverted: the old message restored; the old next step restored; the count's two words swapped; "capitalisation" spelled with a z; "or they collide" appended; the message's trailing colon removed. Each reds both subtests.