Repeated processing of extension TOC entries with add_suffix_to_extensions

[rokm]

This issue is a bit easier to spot now that we divert built-in extensions into lib-dynload directory: when building a onedir application on linux and macOS, we end up with two files in the sys._MEIPASS with oddly mangled filename/path:
_struct/cpython-39-x86_64-linux-gnu/sotruct.cpython-39-x86_64-linux-gnu.so
zlib/cpython-39-x86_64-linux-gnu/soib.cpython-39-x86_64-linux-gnu.so.

It turns out that this is caused by these two extension modules being added to list of dependencies early in the build chain (they are listed among bootstrap modules, probably just in case) and consequently ending up being processed by add_suffix_to_extensions() twice. The reconstructed chain of events is as follows:

• the zlib and _struct extensions are explicitly returned among bootstrap modules by depend.analysis.get_bootstrap_modules(). These are added to PYZ.dependencies.

• the pyz object is passed over to an EXE, whose TOC is extended with those pyz dependencies. At this point, zlib and _struct entries are still intact.

• the EXE then instantiates a PKG, and passes it its TOC. PKG.assemble() performs add_suffix_to_extensions() on the TOC - at this point, the entries are turned into '_struct.cpython-39-x86_64-linux-gnu.so' and 'zlib.cpython-39-x86_64-linux-gnu.so', respectively.

• but in onedir mode, extensions are ignored by PKG, and are instead appended to dependencies.

• the EXE then takes PKG.dependencies, and makes them its own.

• then, COLLECT is instantiated, and is passed the exe instance. It takes the exe's dependencies, and merges them into its own TOC.

• COLLECT.assemble() performs add_suffix_to_extensions() on its TOC, and at this point, the offending two entries have been processed twice, and are now mangled into:
  _struct/cpython-39-x86_64-linux-gnu/sotruct.cpython-39-x86_64-linux-gnu.so
  and
  zlib/cpython-39-x86_64-linux-gnu/soib.cpython-39-x86_64-linux-gnu.so.

So either the assembly pipeline should avoid repeatedly processing entries with add_suffix_to_extensions, or add_suffix_to_extensions should verify that entries do not already have suffix (which is probably easier to do).

In addition, those modules should be explicitly diverted into lib-dynload to follow the behavior from #5604 and avoid creating useless copies.