Fail gracefully when attempting pair destructuring (JuliaLang#49368)

`a => b = x` was interpreted as a function definition. Now it's an error unless you `import Base: =>`

src/module.c

@@ -239,7 +239,7 @@ JL_DLLEXPORT jl_binding_t *jl_get_binding_for_method_def(jl_module_t *m, jl_sym_
  }
  // TODO: we might want to require explicitly importing types to add constructors
  // or we might want to drop this error entirely
- if (!b->imported && (!b2->constp || !jl_is_type(f))) {
+ if (!b->imported && !(b2->constp && jl_is_type(f) && strcmp(jl_symbol_name(var), "=>") != 0)) {
  jl_errorf("invalid method definition in %s: function %s.%s must be explicitly imported to be extended",
  jl_symbol_name(m->name), jl_symbol_name(from->name), jl_symbol_name(var));
  }

test/syntax.jl

@@ -3483,3 +3483,9 @@ end
 # issue #49984
 macro z49984(s); :(let a; $(esc(s)); end); end
 @test let a = 1; @z49984(a) === 1; end
+
+# issues #37783, #39929, #42552, #43379, and #48332
+let x = 1 => 2
+ @test_throws ErrorException @eval a => b = 2
+ @test_throws "function Base.=> must be explicitly imported to be extended" @eval a => b = 2
+end