Bug 1717051: Reuse "requirements" logic in mach_initialize r=ahal

Rather than re-implementing it as `search_path()`, use the existing
`MachEnvRequirements` tool to parse `mach_virtualenv_requirements.txt`

Differential Revision: https://phabricator.services.mozilla.com/D126280

configure.py

@@ -21,6 +21,7 @@ except ImportError:
 
 
 base_dir = os.path.abspath(os.path.dirname(__file__))
+sys.path.insert(0, os.path.join(base_dir, "python", "mach"))
 sys.path.insert(0, os.path.join(base_dir, "python", "mozboot"))
 sys.path.insert(0, os.path.join(base_dir, "python", "mozbuild"))
 sys.path.insert(0, os.path.join(base_dir, "third_party", "python", "six"))