This fixes the following issues:
* "use function" and "use const" inside namespaced code were checking
for conflicts against class imports. Now they always check against
the correct symbol type.
* Symbol conflicts are now always checked within a single file only.
Previously class uses inside namespaced code were checked globally.
This behavior is illegal because symbols from other files are not
visible if opcache is used, resulting in behavioral discrepancies.
Additionally this made the presence/absence of symbol errors dependent
on autoloading order, which is volatile.
* The "single file" restriction is now enforced by collecting defined
symbols inside a separate hash table. Previously it was enforced
(for the non-namespaced case) by comparing the filename of the
symbol declaration. However this is inaccurate if the same filename
is used multiple times, such as may happen if eval() is used.
* Additionally the previous approach relies on symbols being registered
at compile-time, which is not the case for late-bound classes, which
makes the behavior dependent on class declaration order, as well as
opcache (which may cause delayed early-binding).
* Lastly, conflicts are now consistently checked for conditionally
defined symbols. Previously only declaration-after-use conflicts were
checked in this case. Now use-after-declaration conflicts are
detected as well.
