Add support to the Pex for checking if built PEXes are valid Python
zipapps. Currently, Python zipapps must reside in 32 bit zip files due
to limitations of the stdlib `zipimport` module's `zipimporter`; so this
check amounts to a check that the built PEX zip does not use ZIP64
extensions. The check is controlled with a new
`--check {none,warn,error}` option, defaulting to warn.
* Add --check support for zipapps. (2253)