Fixes#20700.
Patch global `WeakMap` before loading plugins to make a `WeakMap` keyed by `context.sourceCode` act as if every file has a unique `context.sourceCode`.
The runtime cost of this patch is very low - for the use case of `WeakMap` keyed by `SourceCode`, it should be *faster* than an actual `WeakMap`, as no methods do actual hash map lookups. The implementation takes care not to leak memory even if plugins use `WeakMap`s extensively or in pathological patterns.
I rebased #20611 on top of this, and after this PR, 100% of `eslint-plugin-mocha`'s conformance tests pass.
It's pretty hacky to be patching globals, but I believe the implementation to be robust. My rationale for this solution is explained in detail in #20700.
overlookmotel