Read debian virtual packages from apt cache

These virtual packages are supported by the rosdep tool, so should be
acceptable for use here as well.

ros_buildfarm/debian_repo.py

@@ -54,4 +54,17 @@ def get_debian_repo_index(debian_repository_baseurl, target, cache_dir):
 
         package_versions[debian_pkg_name] = PlatformPackageDescriptor(version, source_name)
 
+        prefix = 'Provides: '
+        provides = [line[len(prefix):] for line in lines if line.startswith(prefix)]
+        provides = [provide.strip() for line in provides for provide in line.split(',')]
+
+        for provide in provides:
+            provide_version = None
+            if ' ' in provide:
+                provide, provide_spec = provide.split(' ', 1)
+                if provide_spec.startswith('(=') and provide_spec.endswith(')'):
+                    provide_version = provide_spec[2:-1].strip()
+
+            package_versions[provide] = PlatformPackageDescriptor(provide_version, source_name)
+
     return package_versions

test/test_debian_repo.py

@@ -0,0 +1,33 @@
+# Copyright 2024 Open Source Robotics Foundation, Inc.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+import os
+
+from ros_buildfarm.common import PlatformPackageDescriptor
+from ros_buildfarm.common import Target
+from ros_buildfarm.debian_repo import get_debian_repo_index
+
+mock_index_path = os.path.join(os.path.dirname(__file__), 'mock_debian_index')
+
+def test_parse_debian_repo_index(tmpdir):
+    mock_index_url = 'file://' + mock_index_path
+    target = Target('ubuntu', 'noble', 'amd64')
+    index = get_debian_repo_index(mock_index_url, target, str(tmpdir))
+
+    expected = PlatformPackageDescriptor('6.4+20240113-1ubuntu1', 'ncurses')
+    assert expected == index.get('libncurses-dev')
+    assert expected == index.get('libncurses5-dev')
+
+    expected = PlatformPackageDescriptor(None, 'ncurses')
+    assert expected == index.get('ncurses-dev')