debug: Emit log lines for calls to capture_output() and exec_or_return()

Invocations of these two workhorses of the codebase are often useful to
see during debugging and development.

Debug logging is still very primitive in this codebase.

Related-to: <#201>

nextstrain/cli/debug.py

@@ -8,5 +8,13 @@
     parent exceptions in an exception chain are omitted from handled errors.
 """
 from os import environ
+from sys import stderr
 
 DEBUGGING = bool(environ.get("NEXTSTRAIN_DEBUG"))
+
+if DEBUGGING:
+    def debug(*args):
+        print("DEBUG:", *args, file = stderr)
+else:
+    def debug(*args):
+        pass

nextstrain/cli/util.py

@@ -20,6 +20,7 @@
 from textwrap import dedent, indent
 from wcmatch.glob import globmatch, GLOBSTAR, EXTGLOB, BRACE, MATCHBASE, NEGATE
 from .__version__ import __version__
+from .debug import debug
 from .types import RunnerModule, RunnerTestResults
 
 
@@ -276,6 +277,8 @@ def capture_output(argv, extra_env: Mapping = {}):
     If an *extra_env* mapping is passed, the provided keys and values are
     overlayed onto the current environment.
     """
+    debug(f"capture_output({argv!r}, {extra_env!r})")
+
     env = os.environ.copy()
 
     if extra_env:
@@ -308,6 +311,8 @@ def exec_or_return(argv: List[str], extra_env: Mapping = {}) -> int:
 
     ¹ https://bugs.python.org/issue9148
     """
+    debug(f"exec_or_return({argv!r}, {extra_env!r})")
+
     env = os.environ.copy()
 
     if extra_env: