suggest: try to find the implementation of a symbol when def is used (#15555)

* suggest: try to find the implementation of a symbol when def is used

* suggest: return all declarations of the symbol on `def`

nimsuggest/tests/tdef_forward.nim

@@ -0,0 +1,13 @@
+discard """
+$nimsuggest --tester $file
+>def $1
+def;;skProc;;tdef_forward.hello;;proc (): string;;$file;;8;;5;;"";;100
+def;;skProc;;tdef_forward.hello;;proc (): string;;$file;;12;;5;;"";;100
+"""
+
+proc hello(): string
+
+hel#[!]#lo()
+
+proc hello(): string =
+  "Hello"