Modules should compile without including wt_internal.h header file

The lz4 and snappy compression libraries and a number of test programs currently include wt_internal.h. That is bad form, and makes managing wt_internal.h more difficult.

Especially for the compression libraries, since if the compression libraries start using functions from within the WiredTiger compilation (rather than just content from a header file) compilation will start failing on platforms that require library resolution at link time.

The list of non-internal code that includes wt_internal.h is:

bench/wtperf/wtperf.h

ext/compressors/lz4/lz4_compress.c

ext/compressors/snappy/snappy_compress.c

test/packing/intpack-test.c

test/packing/intpack-test2.c

test/packing/intpack-test3.c

test/packing/packing-test.c

test/utility/test_util.i

We should review those usages and figure out how to avoid including wt_internal.h where practical.