Create info module for each DSO

Generates an info module per DSO built with information on the original
dot separated name, the stem and full path to the DSO. The info module
also adds the directory containing the DSO to the DLL path on Windows
once the info module is loaded.

The name for the info module is configurable per DSO and defaults to
`"{stem}_dso.py"`. Setting the name to `None` turns off info moduel
generation for the DSO.

The change does not attempt to create proper package structure to ensure
the info modules are importable, that is up to the user.

src/setuptools_dso/dsocmd.py

@@ -85,11 +85,13 @@ class DSO(_Extension):
                  soversion=None,
                  lang_compile_args=None,
                  dsos=None,
+                 dso_info_module_name="{}_dso.py",
                  **kws):
         _Extension.__init__(self, name, sources, **kws)
         self.lang_compile_args = lang_compile_args or {}
         self.soversion = soversion or None
         self.dsos = dsos or []
+        self.dso_info_module_name = dso_info_module_name
 
 class dso2libmixin:
     def dso2lib_pre(self, ext):
@@ -223,6 +225,7 @@ class build_dso(dso2libmixin, Command):
 
         for dso in self.dsos:
             self.build_dso(dso)
+            self.gen_info_module(dso)
 
     def _name2file(self, dso, so=False):
         """Translate DSO name (eg. "pkg.mod.mylib" into
@@ -230,23 +233,32 @@ class build_dso(dso2libmixin, Command):
         """
         parts = dso.name.split('.')
 
+        parts[-1] = self._name2libname(dso, so)
+
+        return os.path.join(*parts)
+
+    def _name2libname(self, dso, so=False):
+        """
+        Translate DSO name to library name
+
+        For example, DSO "mylib" on Linux would return "libmylib.so".
+        """
+        parts = dso.name.split('.')
+
         if sys.platform == "win32":
-            parts[-1] = parts[-1]+'.dll'
+            return parts[-1]+'.dll'
 
         elif sys.platform == 'darwin':
             if so and dso.soversion is not None:
-                parts[-1] = 'lib%s.%s.dylib'%(parts[-1], dso.soversion)
+                return 'lib%s.%s.dylib'%(parts[-1], dso.soversion)
             else:
-                parts[-1] = 'lib%s.dylib'%(parts[-1],)
+                return 'lib%s.dylib'%(parts[-1],)
 
         else: # ELF
             if so and dso.soversion is not None:
-                parts[-1] = 'lib%s.so.%s'%(parts[-1], dso.soversion)
+                return 'lib%s.so.%s'%(parts[-1], dso.soversion)
             else:
-                parts[-1] = 'lib%s.so'%(parts[-1],)
-
-        return os.path.join(*parts)
-
+                return 'lib%s.so'%(parts[-1],)
 
     def build_dso(self, dso):
         # dso is an instance of DSO
@@ -375,6 +387,57 @@ class build_dso(dso2libmixin, Command):
             if baselib!=solib:
                 self.copy_file(outbaselib, baselib_dst)
 
+    def gen_info_module(self, dso):
+        if dso.dso_info_module_name is None:
+            log.debug("skiping creation of info module")
+            return
+
+        parts = dso.name.split(".")
+
+        info_module_filename = os.path.join(
+            os.path.join(self.build_lib, *parts[:-1]),
+            dso.dso_info_module_name.format(parts[-1]),
+        )
+
+        log.info(
+            "creating info module for {dso_name} at {filename}".format(
+                dso_name=dso.name, filename=info_module_filename
+            )
+        )
+
+        if not self.dry_run:
+            import textwrap
+
+            with open(info_module_filename, "w") as file:
+                file.write(
+                    textwrap.dedent(
+                        """
+                    import os.path
+                    import sys
+                    dir = os.path.dirname(__file__)
+                    if sys.platform == 'win32':
+                        os.add_dll_directory(dir)
+                    dsoname = {dsoname!r}
+                    libname = {libname!r}
+                    filename = os.path.join(dir, libname)
+                    __all__ = ("dsoname", "libname", "filename")
+                    """
+                    ).format(dsoname=dso.name, libname=self._name2libname(dso))
+                )
+
+        if self.inplace:
+            build_py = self.get_finalized_command("build_py")
+            pkg = ".".join(parts[:-1])  # path.to.dso -> path.to
+            pkgdir = build_py.get_package_dir(pkg)  # path.to -> src/path/to
+
+            info_module_dest = os.path.join(
+                pkgdir, os.path.basename(info_module_filename)
+            )
+
+            self.mkpath(os.path.dirname(info_module_dest))
+            self.copy_file(info_module_filename, info_module_dest)
+
+
 class build_ext(dso2libmixin, _build_ext):
 
     # allow build_ext to depend on other commands