[dev.link] cmd/link: return package path in SymFile

[dev.link] cmd/link: return package path in SymFile

SymFile, derived from sym.Symbol.File, is supposed to return the
package path, instead of the file name (arguably the name is
confusing). Make it so, and rename it to SymPkg.

Change-Id: I67bcd12f67cea271f2a2ce3c5724e5d228f5b2f0
Reviewed-on: https://go-review.googlesource.com/c/go/+/227481
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Jeremy Faller <jeremy@golang.org>
Reviewed-by: Than McIntosh <thanm@google.com>