Clear any prior vendorings when (re)vendoring.

This allows for re-vendored distributions losing files as well as changing
the set of vendored distributions altogether.

Fixes pex-tool#1096

pex/vendor/__init__.py

@@ -39,6 +39,12 @@ class VendorSpec(
 
     ROOT = _root()
 
+    _VENDOR_DIR = "_vendored"
+
+    @classmethod
+    def vendor_root(cls):
+        return os.path.join(cls.ROOT, *(_PACKAGE_COMPONENTS + [cls._VENDOR_DIR]))
+
     @classmethod
     def pinned(cls, key, version, rewrite=True):
         return cls(
@@ -61,7 +67,7 @@ def vcs(cls, url, rewrite=True):
 
     @property
     def _subpath_components(self):
-        return ["_vendored", self.key]
+        return [self._VENDOR_DIR, self.key]
 
     @property
     def relpath(self):

pex/vendor/__main__.py

@@ -15,7 +15,7 @@
 
 from pex import third_party
 from pex.common import safe_delete, safe_rmtree
-from pex.vendor import iter_vendor_specs
+from pex.vendor import VendorSpec, iter_vendor_specs
 
 
 class ImportRewriter(object):
@@ -285,9 +285,10 @@ def vendorize(root_dir, vendor_specs, prefix):
         print("Usage: {}".format(sys.argv[0]), file=sys.stderr)
         sys.exit(1)
 
-    root_directory = os.path.realpath(os.path.join(os.path.dirname(__file__), "..", ".."))
+    root_directory = VendorSpec.ROOT
     import_prefix = third_party.import_prefix()
     try:
+        safe_rmtree(VendorSpec.vendor_root())
         vendorize(root_directory, list(iter_vendor_specs()), import_prefix)
         sys.exit(0)
     except VendorizeError as e: