Taskgraph doesn't validate index routes well enough

[Eijebong]

Taskcluster doesn't validate index routes itself: taskcluster/taskcluster#6266
Taskgraph has a verification pass for these since #722

However, it only checks for / because the original issue only mentioned those.

I have the case where I tried having a and a + in my routes, which taskgraph happily accepted until I started trying to eager index said tasks and saw it fail with URL parameter 'namespace' given as 'ap.archipelago-index.fuzz.pr.-1.ape_escape_3.1.0.80-exp+hotfix1.default.latest' must match regular expression: '/^([a-zA-Z0-9_!~*'()%-]+\.)*[a-zA-Z0-9_!~*'()%-]+$/'

The verify_index_route should get extended to verify that we match that regex instead of just checking for /