Define publicity at a per-binding level, not per-symbol

[LilithHafner]

Symbols don't belong to modules and can't be marked public. The statement public Fix in Base does not make the symbol :Fix public everywhere. My package's Package.Fix may still be private. It's bindings that are marked this way:

julia/src/julia.h

Lines 695 to 705 in fa9478b

	typedef struct _jl_binding_t {
	JL_DATA_TYPE
	jl_globalref_t *globalref; // cached GlobalRef for this binding
	_Atomic(jl_value_t*) value;
	_Atomic(jl_binding_partition_t*) partitions;
	uint8_t declared:1;
	uint8_t exportp:1; // `public foo` sets `publicp`, `export foo` sets both `publicp` and `exportp`
	uint8_t publicp:1; // exportp without publicp is not allowed.
	uint8_t deprecated:2; // 0=not deprecated, 1=renamed, 2=moved to another package
	uint8_t padding:3;
	} jl_binding_t;

[LilithHafner]

From #57093 (review)