build: use golangci-lint (#20295)

* build: use golangci-lint

This changes build/ci.go to download and run golangci-lint instead
of gometalinter.

* core/state: fix unnecessary conversion

* p2p/simulations: fix lock copying (found by go vet)

* signer/core: fix unnecessary conversions

* crypto/ecies: remove unused function cmpPublic

* core/rawdb: remove unused function print

* core/state: remove unused function xTestFuzzCutter

* core/vm: disable TestWriteExpectedValues in a different way

* core/forkid: remove unused function checksum

* les: remove unused type proofsData

* cmd/utils: remove unused functions prefixedNames, prefixFor

* crypto/bn256: run goimports

* p2p/nat: fix goimports lint issue

* cmd/clef: avoid using unkeyed struct fields

* les: cancel context in testRequest

* rlp: delete unreachable code

* core: gofmt

* internal/build: simplify DownloadFile for Go 1.11 compatibility

* build: remove go test --short flag

* .travis.yml: disable build cache

* whisper/whisperv6: fix ineffectual assignment in TestWhisperIdentityManagement

* .golangci.yml: enable goconst and ineffassign linters

* build: print message when there are no lint issues

* internal/build: refactor download a bit

.golangci.yml

@@ -0,0 +1,45 @@
+# This file configures github.com/golangci/golangci-lint.
+
+run:
+  timeout: 2m
+  tests: true
+  # default is true. Enables skipping of directories:
+  #   vendor$, third_party$, testdata$, examples$, Godeps$, builtin$
+  skip-dirs-use-default: true
+
+linters:
+  disable-all: true
+  enable:
+    - deadcode
+    - goconst
+    - goimports
+    - gosimple
+    - govet
+    - ineffassign
+    - misspell
+    # - staticcheck
+    - unconvert
+    # - unused
+    - varcheck
+
+linters-settings:
+  gofmt:
+    simplify: true
+  goconst:
+    min-len: 3 # minimum length of string constant
+    min-occurrences: 6 # minimum number of occurrences
+
+issues:
+  exclude-rules:
+    - path: crypto/blake2b/
+      linters:
+        - deadcode
+    - path: crypto/bn256/cloudflare
+      linters:
+        - deadcode
+    - path: p2p/discv5/
+      linters:
+        - deadcode
+    - path: core/vm/instructions_test.go
+      linters:
+        - goconst