Fix tab-completion for :break (#17989)

In tab-completion for the `:break` command, only those
identifiers should be shown, that are accepted in the
`:break` command. Hence these identifiers must be

- defined in an interpreted module
- top-level
- currently in scope
- listed in a `ModBreaks` value as a possible breakpoint.

The identifiers my be qualified or unqualified.

To get all possible top-level breakpoints for tab-completeion
with the correct qualification do:

1. Build the  list called `pifsBreaks` of all pairs of
(Identifier, module-filename) from the `ModBreaks` values.
Here all identifiers are unqualified.

2. Build the list called `pifInscope` of all pairs of
(Identifiers, module-filename) with identifiers from
the `GlobalRdrEnv`. Take only those identifiers that are
in scope and have the  correct prefix.
Here the identifiers may be qualified.

3. From the `pifInscope` list seclect all pairs that can be
found in the `pifsBreaks` list, by comparing only the
unqualified part of the identifier.
The remaining identifiers can be used for tab-completion.

This ensures, that we show only identifiers, that can be used
in a `:break` command.

1 files changed, 1 insertions, 0 deletions

diff --git a/testsuite/tests/ghci.debugger/scripts/all.T b/testsuite/tests/ghci.debugger/scripts/all.T
index db597a455f..12fe420363 100644
--- a/testsuite/tests/ghci.debugger/scripts/all.T
+++ b/testsuite/tests/ghci.debugger/scripts/all.T
@@ -125,3 +125,4 @@ test('T16700', normal, ghci_script, ['T16700.script'])
 
 test('break029', extra_files(['break029.hs']), ghci_script, ['break029.script'])
 test('T2215', normal, ghci_script, ['T2215.script'])
+test('T17989', normal, ghci_script, ['T17989.script'])