[nimsuggest] fix def call on identifier 2 times on the line (#20228)

- apparently TLineInfo's implementation of `==` ignores the column. After I fixed
the code to use exact TLineInfo comparison I fixed several other issues hidden
by that issue.

- Replaced `tuple[sym, info]` with `SymInfoPair`

nimsuggest/tests/tv3_definition.nim

@@ -0,0 +1,9 @@
+
+let foo = 30
+let bar = foo + fo#[!]#o + foo
+
+discard """
+$nimsuggest --v3 --tester $file
+>def $1
+def	skLet	tv3_definition.foo	int	$file	2	4	""	100
+"""