Filename case-consistency rule

[atsu85]

@dagtveit stumbled upon issue related to having two instances of the same module that was intended to be singleton:

I am trying to share a service class across nested custom elements. but the service class dosnt have the same values.

He found out that it was caused by inconsistent letter-cases

it was due to that i wrote small d in documentData in one of the imports and big D in the other one. laaaaaaaaalllalalalal hhehehehe

It reminded me, that I had made the same mistake once, and it took me also a bit time to figure out the problem.
It would be nice, if the linter would detect this issue as well.

[MeirionHughes]

May be better to issue an warning if the case of the file name is not exactly the same as what was requested. For one, that in itself could be a problem on linux installations set up to be case sensitive.

Either way, its easier to check for this in the reworked code.