go/types: in TestCheck/issues.src, import regexp/syntax instead of cmd/compile/internal/syntax

TestCheck/issues.src was failing after running
	rm -r $(go env GOROOT)/pkg/*/cmd
as the builders do when building binary releases.

For users who write programs that depend on go/types, it should be
reasonable for end users to run the tests for go/types as part of 'go
test all', and those tests should pass even if they installed Go from
a binary release.

The test case in issues.src was importing cmd/compile/internal/syntax
in order to check the reported package name.

I tried to fix the problem by having the test import from source
instead of from export data. Unfortunately, that changed the behavior
under test: the go/types.Package.Imports reports (and is documented to
report) a different set of imported packages when loading from source
as compared to when loading from export data.

For this particular test, after CL 313035 that difference resulted in
go/types treating the "syntax" name as ambiguous when importing from
source, because a transitive dependency on "regexp/syntax" is found
when loading from source but omitted when loading from export data.

The simple fix to make the package unambiguous again is to adapt the
test to import regexp/syntax directly. That not only makes the package
unambiguous with all importers, but also avoids depending on a
cmd-internal package that cannot be loaded from export data in binary
distributions of the Go toolchain.

For #43232

Change-Id: Iba45a680ea20d26daa86ac538fd8f1938e8b73ab
Reviewed-on: https://go-review.googlesource.com/c/go/+/330431


Trust: Bryan C. Mills <bcmills@google.com>
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>