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='2022-01-01 15:10:12 +0100'>2022-01-01</span></td><td><a href='/guix/commit/gnu/system/pam.scm?id=671e6a81804f264ddcdd6fe7579644404da079b8'>system: Allow 'chfn' to change the user's full name.</a><span class='msg-avail'>...<span class='msg-tooltip'>Fixes &lt;https://issues.guix.gnu.org/52539&gt;.
Reported by Jacob First &lt;jacob.first@member.fsf.org&gt;.

* gnu/build/accounts.scm (allocate-passwd): Add comment as to why
'real-name' is taken from PREVIOUS.  Add (not system?) to the
condition.
* gnu/system.scm (operating-system-etc-service) &lt;login.defs&gt;: Add
"CHFN_RESTRICT".
* gnu/system.scm (%setuid-programs): Add "chfn".
* gnu/system/pam.scm (base-pam-services): Add "chfn".
* doc/guix.texi (User Accounts): Document it.
</span></span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2021-08-17 01:10:25 +0300'>2021-08-17</span></td><td><a href='/guix/commit/gnu/system/pam.scm?id=b948ab8b56ac238e7a774625b8f971d28aee6055'>services: slim: Add pam-gnupg support.</a><span class='msg-avail'>...<span class='msg-tooltip'>* gnu/system/pam.scm (unix-pam-service): Add account and session PAM entries
for pam-gnupg.  Don't pass "#f" to "allow-root?" argument, because "lambda*"
already does this by default.
* doc/guix.texi (X Window): Document this.
* gnu/services/xorg.scm (&lt;slim-configuration&gt;)[gnupg?]: New record field.
(slim-pam-service): Pass "#:gnupg?" argument to "unix-pam-service".
</span></span></td><td>Oleg Pykhalov</td></tr>
<tr><td><span title='2020-05-08 16:30:50 +0200'>2020-05-08</span></td><td><a href='/guix/commit/gnu/system/pam.scm?id=dd0804c67e99914eecc471fdb92f4220425a1cbc'>services: Add descriptions.</a><span class='msg-avail'>...<span class='msg-tooltip'>* gnu/services/dbus.scm (polkit-service-type)[description]: New field.
* gnu/services/dict.scm (dicod-service-type)[description]: New field.
* gnu/services/dns.scm (knot-service-type)[description]: New field.
* gnu/services/networking.scm (dhcpd-service-type)[description]: New field.
* gnu/services/shepherd.scm (shepherd-root-service-type)[description]:
New field.
* gnu/services/xorg.scm (slim-service-type)[description]: New field.
(screen-locker-service-type)[description]: New field.
* gnu/system/pam.scm (pam-root-service-type)[description]: New field.
* gnu/system/shadow.scm (account-service-type)[description]: New field.
</span></span></td><td>Ludovic Courtès</td></tr>