[dev.boringcrypto] crypto/..., go/build: align deps test with standard rules

[dev.boringcrypto] crypto/..., go/build: align deps test with standard rules

One annoying difference between dev.boringcrypto and master is that
there is not a clear separation between low-level (math/big-free)
crypto and high-level crypto, because crypto/internal/boring imports
both encoding/asn1 and math/big.

This CL removes both those problematic imports and aligns the
dependency rules in the go/build test with the ones in the main
branch.

To remove encoding/asn1, the crypto/internal/boring APIs change to
accepting and returning encoded ASN.1, leaving crypto/ecdsa to do the
marshaling and unmarshaling, which it already contains code to do.

To remove math/big, the crypto/internal/boring package defines
type BigInt []uint, which is the same representation as a big.Int's
internal storage. The new package crypto/internal/boring/bbig provides
conversions between BigInt and *big.Int. The boring package can then
be in the low-level crypto set, and any package needing to use bignum
APIs (necessarily in the high-level crypto set) can import bbig to
convert.

To simplify everything we hide from the test the fact that
crypto/internal/boring imports cgo. Better to pretend it doesn't and
keep the prohibitions that other packages like crypto/aes must not use
cgo (outside of BoringCrypto).

	$ git diff origin/master src/go/build/deps_test.go
	diff --git a/src/go/build/deps_test.go b/src/go/build/deps_test.go
	index 6ce872e297..a63979cc93 100644
	--- a/src/go/build/deps_test.go
	+++ b/src/go/build/deps_test.go
	@@ -402,9 +402,13 @@ var depsRules = `
	 	NET, log
	 	< net/mail;

	+	NONE < crypto/internal/boring/sig;
	+	sync/atomic < crypto/internal/boring/fipstls;
	+	crypto/internal/boring/sig, crypto/internal/boring/fipstls < crypto/tls/fipsonly;
	+
	 	# CRYPTO is core crypto algorithms - no cgo, fmt, net.
	 	# Unfortunately, stuck with reflect via encoding/binary.
	-	encoding/binary, golang.org/x/sys/cpu, hash
	+	crypto/internal/boring/sig, encoding/binary, golang.org/x/sys/cpu, hash
	 	< crypto
	 	< crypto/subtle
	 	< crypto/internal/subtle
	@@ -413,6 +417,8 @@ var depsRules = `
	 	< crypto/ed25519/internal/edwards25519/field, golang.org/x/crypto/curve25519/internal/field
	 	< crypto/ed25519/internal/edwards25519
	 	< crypto/cipher
	+	< crypto/internal/boring
	+	< crypto/boring
	 	< crypto/aes, crypto/des, crypto/hmac, crypto/md5, crypto/rc4,
	 	  crypto/sha1, crypto/sha256, crypto/sha512
	 	< CRYPTO;
	@@ -421,6 +427,7 @@ var depsRules = `

	 	# CRYPTO-MATH is core bignum-based crypto - no cgo, net; fmt now ok.
	 	CRYPTO, FMT, math/big, embed
	+	< crypto/internal/boring/bbig
	 	< crypto/rand
	 	< crypto/internal/randutil
	 	< crypto/ed25519
	@@ -443,7 +450,8 @@ var depsRules = `
	 	< golang.org/x/crypto/hkdf
	 	< crypto/x509/internal/macos
	 	< crypto/x509/pkix
	-	< crypto/x509
	+	< crypto/x509;
	+	crypto/internal/boring/fipstls, crypto/x509
	 	< crypto/tls;

	 	# crypto-aware packages
	@@ -653,6 +661,9 @@ func findImports(pkg string) ([]string, error) {
	 	}
	 	var imports []string
	 	var haveImport = map[string]bool{}
	+	if pkg == "crypto/internal/boring" {
	+		haveImport["C"] = true // kludge: prevent C from appearing in crypto/internal/boring imports
	+	}
	 	fset := token.NewFileSet()
	 	for _, file := range files {
	 		name := file.Name()

For #51940.

Change-Id: I26fc752484310d77d22adb06495120a361568d04
Reviewed-on: https://go-review.googlesource.com/c/go/+/395877
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Roland Shoemaker <roland@golang.org>
Run-TryBot: Russ Cox <rsc@golang.org>