fix: prefix external command output for better log clarity

Most log output uses the package name as a prefix (via FromagerLogRecord)
so it is possible to tell which package is being processed when a message
is logged. However, the output of external commands (like uv) was logged
without any visual indication that it came from an external command rather
than fromager itself, making logs harder to read.

This fix adds a visual prefix ('  > ') to each line of external command
output using str.replace(), then logs the entire output in a single call.

Benefits:
- Visual clarity: The '> ' prefix clearly distinguishes external command
  output from fromager's own log messages
- Parallel builds: Logging the entire output as one block prevents line
  interleaving when building multiple packages in parallel
- Package context: The package name prefix is still applied to the whole
  output block via FromagerLogRecord
- Simple implementation: Uses a single str.replace() call

Example output:
  numpy: command failed with exit code 1: uv pip install numpy
  numpy:   > Resolving dependencies...
    > Error: Could not find package

Fixes: #753

Co-authored-by: Claude <claude@anthropic.com>

Signed-off-by: Ioannis Angelakopoulos <iangelak@redhat.com>

Author: iangelak

src/fromager/external_commands.py

@@ -99,7 +99,16 @@ def run(
         )
         output = completed.stdout.decode("utf-8") if completed.stdout else ""
     if completed.returncode != 0:
-        logger.error("%s failed with %s", cmd, output)
+        # Log the command failure first
+        logger.error(
+            "command failed with exit code %d: %s",
+            completed.returncode,
+            " ".join(shlex.quote(str(s)) for s in cmd),
+        )
+        # Log command output with visual prefix to distinguish from fromager output
+        if output:
+            logger.error("  > " + output.replace("\n", "\n  > "))
+
         err_type = subprocess.CalledProcessError
         if network_isolation:
             # Look for a few common messages that mean there is a network
@@ -113,5 +122,9 @@ def run(
                 if substr in output:
                     err_type = NetworkIsolationError
         raise err_type(completed.returncode, cmd, output)
-    logger.debug("output: %s", output)
+
+    # Log command output with visual prefix to distinguish from fromager output
+    if output:
+        logger.debug("  > " + output.replace("\n", "\n  > "))
+
     return output

tests/test_external_commands.py

@@ -29,7 +29,10 @@ def test_external_commands_log_file(tmp_path):
     assert "test\n" == file_contents
 
 
-@mock.patch("subprocess.run", return_value=mock.Mock(returncode=0))
+@mock.patch(
+    "subprocess.run",
+    return_value=mock.Mock(returncode=0, stdout=b"test output\n"),
+)
 @mock.patch(
     "fromager.external_commands.network_isolation_cmd",
     return_value=["/bin/unshare", "--net", "--map-current-user"],