*.php files are ignored by Git and a better practice might be to rename
PHP included files for tests.

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.

This implements a reduced variant of #1226 with just the following
change:

-Fatal error: Uncaught exception 'EngineException' with message 'Call to private method foo::bar() from context ''' in %s:%d
+Fatal error: Uncaught EngineException: Call to private method foo::bar() from context '' in %s:%d

The '' wrapper around messages is very weird if the exception
message itself contains ''. Futhermore having the message wrapped
in '' doesn't work for the "and defined" suffix of
TypeExceptions.

TypeException stays as-is for now because it uses messages that are
incompatible with the way exception messages are displayed.

closure_038.phpt and a few others now show that we're generating
too many exceptions for compound operations on undefined properties
-- this needs to be fixed in a followup.