taskgraph should raise and exception if index routes have a `/` in them

[bhearsum]

According to the Taskcluster documentation, slashes are forbidden in index routes:

Note that slashes (/) aren’t allowed in index paths.

Taskgraph will happily create them though, and when these routes are used as part of caches, it ends up with cache routes that never work (and tasks that never get optimized by replacement).

[bhearsum]

Index routes for cached tasks are defined around

taskgraph/src/taskgraph/util/cached_tasks.py

Lines 57 to 62 in dc5b490

	index_routes = []
	min_level = int(config.params["level"])
	for level in reversed(range(min_level, 4)):
	subs["level"] = level
	index_routes.append(TARGET_CACHE_INDEX.format(**subs))

; we'll want to check them around the same place.

A new test case or two will need to be added in

taskgraph/test/test_util_cached_tasks.py

Line 181 in dc5b490

def test_add_optimization(

. That function will need to be tweaked to allow setting the cache_name per test so that the new test(s) can include a / in them.