.tap() `before` parameter do not respects existing befores

[Rulexec]

Hello. I have three plugins:

• A depends on B
• B depends on C
• C

They tap some hook with same stage in the following order:

• A, B, C

Then inside

tapable/lib/Hook.js

Line 136 in 9d647f8

_insert(item) {

they got position incorrectly as:

• B, A, C

It happens because current logic looks like:

• Adding A, no taps, just add
• Adding B, B has before, so if (before.size > 0) allows to skip A and be inserted in the start of taps (e.g. order now is B, A)
• Adding C to the top, since it has no before rules

[Rulexec]

Also it's will be good to throw exception if circular dependency is detected.