[dev.regabi] cmd/compile: initialize importMap lazily
[dev.regabi] cmd/compile: initialize importMap lazily

This sets up the next CL, moving importMap to a global zeroed struct.

Change-Id: I1acc91b440d3da6e28fb32bd275fb3cd36db4e97
Reviewed-on: https://go-review.googlesource.com/c/go/+/272046
Trust: Russ Cox <5ad239cb8a44f659eaaee0aa1ea5b94947abe557@golang.org>
Reviewed-by: Matthew Dempsky <e56ac2c500064424afac424d4b5e53bd0570a731@google.com>

golang/go,golang/go,CAFxX/go,CAFxX/go,nawawi/go,cloudflare/go,cloudflare/go,cloudflare/go,CAFxX/go,nawawi/go,golang/go,cloudflare/go,cloudflare/go,golang/go,cloudflare/go,CAFxX/go,nawawi/go,cloudflare/go,nawawi/go,CAFxX/go,golang/go,nawawi/go,CAFxX/go,nawawi/go,golang/go,nawawi/go,CAFxX/go,CAFxX/go,nawawi/go,golang/go,golang/go,cloudflare/go