Package name specification needs refinement

Right now, the specification says that "package [...] names may consist of ASCII letters, numbers, hyphens (-), and underscores (_), and may not contain forward-stroke (/) or at-sign (@)." This is both too restrictive and too liberal; we should probably also explicitly exclude all characters not allowed in FAT file names (`<`, `>`, `:`, `"`, `\`, `|`, `?`, `*`), and we should *add* `+` and `.` to the set of officially blessed characters, as these are used in existing package names. (As a bonus, this also makes the naming schema nearly identical to what CMake considers acceptable target names.)

The issue names only the package-name specification, not a file or test. Start by locating that specification and reviewing the four comments; done means the allowed-character rules explicitly exclude the listed FAT filename characters and include + and . while matching the project’s agreed wording.

Written by the indexing model from the issue text.