Add a dependencies command, which dumps the list of dependencies

This is useful for automating things like installing only a project's
dependencies, in CI.

bork/api.py

@@ -40,6 +40,12 @@ def clean():
             try_delete(name)
 
 
+def dependencies():
+    """Get the list of dependencies."""
+    from pep517 import meta
+    return meta.load('.').metadata.get_all('Requires-Dist')
+
+
 def download(package, release_tag, file_pattern, directory):
     if file_pattern is None or len(file_pattern) == 0:
         raise ValueError('file_pattern must be non-empty.')

bork/cli.py

@@ -40,6 +40,14 @@ def clean():
     api.clean()
 
 
+@cli.command()
+@click.option('-o', '--output', type=click.File('w'), default='-',
+              help='File in which to save the list of dependencies.')
+def dependencies(output):
+    for dep in api.dependencies():
+        print(dep, file=output)
+
+
 @cli.command()
 @click.option('--files', default='*.pyz',
               help='Comma-separated list of filenames to download. Supports '