cmd/compile: better error for malformed packages
cmd/compile: better error for malformed packages

Fixes #14270.

Change-Id: Iaf2496a24b1aba443391b90951cef472add73f13
Reviewed-on: https://go-review.googlesource.com/45950
Run-TryBot: Matthew Dempsky <e56ac2c500064424afac424d4b5e53bd0570a731@google.com>
Reviewed-by: Robert Griesemer <a6752097faebf1f02d80107d77a9be009089afe3@golang.org>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>

polyverse-security/go,vsekhar/elastic-go,codestation/go,codestation/go,josharian/go,polyverse-security/go,Cofyc/go,akutz/go,CAFxX/go,nawawi/go,nawawi/go,josharian/go,nawawi/go,golang/go,vsekhar/elastic-go,codestation/go,CAFxX/go,golang/go,Cofyc/go,golang/go,josharian/go,codestation/go,cloudflare/go,CAFxX/go,josharian/go,Cofyc/go,vsekhar/elastic-go,polyverse-security/go,CAFxX/go,Cofyc/go,cloudflare/go,CAFxX/go,akutz/go,cloudflare/go,vsekhar/elastic-go,golang/go,polyverse-security/go,Cofyc/go,polyverse-security/go,cloudflare/go,golang/go,cloudflare/go,vsekhar/elastic-go,akutz/go,polyverse-security/go,nawawi/go,CAFxX/go,CAFxX/go,Cofyc/go,polyverse-security/go,golang/go,codestation/go,akutz/go,nawawi/go,christopher-henderson/Go,nawawi/go,cloudflare/go,nawawi/go,vsekhar/elastic-go,golang/go,josharian/go,josharian/go,akutz/go,codestation/go,golang/go,akutz/go,codestation/go,cloudflare/go,cloudflare/go,polyverse-security/go,codestation/go,vsekhar/elastic-go,Cofyc/go,josharian/go,nawawi/go,CAFxX/go,Cofyc/go,vsekhar/elastic-go,akutz/go,josharian/go,akutz/go