Variables: Make scope name format more strict

[medikoo]

Use case description

Currently allowed scope name format is relaxed to e.g. support . or -. To it's properly gather notation as cf.us-east-1:foo.

Still this scope name notation is already deprecated, and even scopes with such named are not really resolved by new resolver (resolution is then fallback to old resolver).

Scope name format is constructed that way to fully detect all variables written in old notation (so we have full information needed to decided whether old resolver should be invoked or not).

Currently supporting such extended format introduced side effect, as in some cases we catch AWS vars as Serverless Vars, as reported here: https://forum.serverless.com/t/brave-new-variable-resolver-world/14704

With next major we don't have to do that, and we can narrow supported format to [a-z][a-zA-Z0-9.]*. (I'd still keep support for . to allow providing solutions as e.g. proposed here: #5202)

Proposed solution

Update supported format to [a-z][a-zA-Z0-9.]*