Work around trytond.module not being a real namespace module.

Solution is to add all trytond module's locations to
trytond.modules._path__. This will make trytond.module behave much
like a namespace module.
Adding to __path__ is done in update_egg_modules() to ensure __path__
is updated whenever the list of egg modules is updated.

*** a/trytond/modules/__init__.py	1970-01-01 01:00:01.000000000 +0100
--- b/trytond/modules/__init__.py	2021-12-02 18:12:15.385101986 +0100
***************
*** 38,43 ****
--- 38,46 ----
          import pkg_resources
          for ep in pkg_resources.iter_entry_points('trytond.modules'):
              EGG_MODULES[ep.name] = ep
+             path = os.path.join(ep.dist.location, 'trytond', 'modules')
+             if not path in __path__ and os.path.isdir(path):
+                 __path__.append(path)
      except ImportError:
          pass
  
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<tr><td><span title='2023-07-20 10:11:26 +0200'>2023-07-20</span></td><td><a href='/guix/commit/gnu/packages/libunistring.scm?id=d4306847bf8736bf60a2ff435fd2d8ef763f769c'>Revert "gnu: libunistring: Fix make check for the Hurd."</a><span class='msg-avail'>...<span class='msg-tooltip'>This reverts commit 12e6a5923812abf180f03bea9e07dd99244a3fcd, which used
XFAIL_TESTS to have 'make check' pass with failing tests "test-perror2"
"test-strerror_r" that now pass with glibc-2.37 and libc-for-target in
commencement.
</span></span></td><td>Janneke Nieuwenhuizen</td></tr>
<tr><td><span title='2023-05-31 07:51:28 +0200'>2023-05-31</span></td><td><a href='/guix/commit/gnu/packages/libunistring.scm?id=12e6a5923812abf180f03bea9e07dd99244a3fcd'>gnu: libunistring: Fix make check for the Hurd.</a><span class='msg-avail'>...<span class='msg-tooltip'>* gnu/packages/libunistring.scm (libunistring)[arguments]: Set XFAIL_TESTS
with test-perror2, and test-strerror_r when building to the Hurd.
</span></span></td><td>Janneke Nieuwenhuizen</td></tr>
<tr><td><span title='2022-05-28 22:01:09 +0300'>2022-05-28</span></td><td><a href='/guix/commit/gnu/packages/libunistring.scm?id=3acb615a3828207d55f52873b669c6917a16e10d'>gnu: libunistring: Rewrite using gexps.</a><span class='msg-avail'>...<span class='msg-tooltip'>* gnu/packages/libunistring.scm (libunistring)[arguments]: Rewrite using
gexps. Remove trailing #t.
</span></span></td><td>Efraim Flashner</td></tr>
<tr><td><span title='2022-05-28 22:01:09 +0300'>2022-05-28</span></td><td><a href='/guix/commit/gnu/packages/libunistring.scm?id=e7fa95dc46e984cdd3b47d89405b8ba304e73dec'>gnu: libunistring: Update to 1.0.</a><span class='msg-avail'>...<span class='msg-tooltip'>* gnu/packages/libunistring.scm (libunistring): Update to 1.0.
[license]: Update in accordance with upstream.
</span></span></td><td>Efraim Flashner</td></tr>