Fixes Homebrew/homebrew#30472.

Fixes Homebrew/homebrew#30494.

name is the user-provided string and may be an alias, path, or URL.

rack cannot be nil at the point where FormulaUnavailableError is raised,
so we don't need a branch to deal with that.

dirs will be empty if the rack directory does not exist.

Only link_dir needs to be protected

Only display —-help for internal commands and not for external ones;
they can handle the flag themselves.

Closes Homebrew/homebrew#26675.
References Homebrew/homebrew#26755.
Closes Homebrew/homebrew#30300.

Eventually we'll move Contributions to a tap too.

Closes Homebrew/homebrew#30298.

The error handling depends on side effects of `require "net/https"`,
so it should be wrapped in an explicit begin block.

cf. Homebrew/homebrew#30407.