packages: fix package prefix read/print issue.

Suppose that we have two symbols of the same name, in two
packages: foo:sym and bar:sym.  Suppose that the bar package
has foo in its package fallback list, and suppose bar is the
current package. Then bar:sym prints without a package prefix,
as just sym. However, this is potentially ambiguous.  Suppose
that bar:sym is written to a file as just sym.  Then later the
file is read into a fresh image in a situation in which
bar:sym has not yet been interned, but foo:sym already exists.
In this situation, sym will just resolve to foo:sym.

The printer must detect this ambiguous situation. If a symbol
is present in a package, but a same-named symbol is in the
fallback list; or if a symbol is visible in the fallback list,
but a same-named symbol is present in the package, then
a package prefix should be printed.

* lib.c (symbol_needs_prefix): New function.
(unquote_star_check, obj_print_impl): Use symbol_needs_prefix
rather than symbol_visible.

* lib.h (symbol_needs_prefix): Declared.

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