Add tool classpath for ./pants scalafix (#6926)

### Problem

Same as in pr #6922
In order to run custom scalafix rules that are distributed in .jar form, they must be added to scalafix's '--tool-classpath' command line argument. Currently there is no way to set this argument from pants.

### Solution

Add scalafix-tool-classpath via register_jvm_tool to ./pants scalafix, with default empty classpath so you don't need to add a target if you don't want.

### Result

scalafix_tool_classpath can be set in pants.ini or on the command line, and scalafix will be able to load your precompiled custom rules.

src/python/pants/backend/jvm/tasks/scalafix.py

@@ -36,6 +36,7 @@ def register_options(cls, register):
                           classpath=[
                             JarDependency(org='ch.epfl.scala', name='scalafix-cli_2.11.11', rev='0.5.2'),
                           ])
+    cls.register_jvm_tool(register, 'scalafix-tool-classpath', classpath=[])
 
   @classmethod
   def target_types(cls):
@@ -58,6 +59,9 @@ def _compute_classpath(self, targets):
 
   def invoke_tool(self, absolute_root, target_sources):
     args = []
+    tool_classpath = self.tool_classpath('scalafix-tool-classpath')
+    if tool_classpath:
+      args.append('--tool-classpath={}'.format(':'.join(tool_classpath)))
     if self.get_options().semantic:
       # If semantic checks are enabled, pass the relevant classpath entries for these
       # targets.