fix: Look for active breaks in right ModBreaks

[alt-romes]

In `getActiveBreakpoints`, we were incorrectly looking up the occurence
module of an internal breakpoint id (`getBreakSourceMod`) on the
`InternalModBreaks` of a different module (`readIModBreaksMaybe` of the
argument module, not the internal breakpoint id one)!

That is just plain incorrect...

The `activeBreakpoints` map stores the active internal-breakpoint-ids.
To get the active breakpoints at module A, we find the *occurrence
module* for every internal breakpoint id in `activeBreakpoints` and keep
the ones whose occurrence module matches the argument module (recall: a
breakpoint defined in B may be inlined in A, which means
`getBreakSourceMod` will return `A` -- and that's what we want to match
against the argument module)

(See also Note [ModBreaks vs InternalModBreaks] and Note [Breakpoint identifiers] in GHC)

Fixes #297