Introduce Go modules

[bzz]

This fixes #170 by adding Modules support.

• go.sum and go.mod added
• github import path used instead of gopkg.in
• CI forced to use modules \w 2 latest Go versions, no go get
• go test all failure 1 heuristics.go
• go test all failure 2 TestCrashDumpsAllThreads
• CI: fix cgo profiles for newer go compiler versions
• update to newest release of fixed compiler
• bump major version to /v2

[bzz]

$ go test all

# github.com/src-d/enry/internal/code-generator/generator
internal/code-generator/generator/heuristics.go:54:17: Errorf format %q has arg patterns[ext] of wrong type []*github.com/src-d/enry/internal/code-generator/generator.LanguagePattern

FAIL	github.com/src-d/enry/internal/code-generator/generator [build failed]

--- FAIL: TestCrashDumpsAllThreads (0.03s)
    crash_unix_test.go:70: building source: exit status 1
        go: cannot find main module; see 'go help modules'
FAIL
FAIL	runtime	161.287s

Although these are not sorted out yet, early feedback is very appreciated!

[bzz]

The only failure on go test all is now a

--- FAIL: TestCrashDumpsAllThreads (0.04s)
    crash_unix_test.go:70: building source: exit status 1
        go: cannot find main module; see 'go help modules'
FAIL
FAIL	runtime	162.087s

which does not seem relevant.

One last thing left is - fix a CGO profiles on CI that seems to fail due to what seems to be some compiler/linker version missmatch

# github.com/src-d/enry.test
/home/travis/.gimme/versions/go1.12.3.linux.amd64/pkg/tool/linux_amd64/link: running gcc failed: exit status 1
/usr/bin/ld: /tmp/go-link-029942625/000006.o: unrecognized relocation (0x2a) in section `.text'
/usr/bin/ld: final link failed: Bad value
collect2: error: ld returned 1 exit status

[creachadair]

which does not seem relevant.

It looks relevant to me—the error is from a module-aware build, not being able to identify the main module. It is probably a configuration issue rather than a code bug, though. This is typical of what you get if either (a) you build inside $GOPATH with modules not enabled (e.g., Go 1.11.x without GO111MODULE=on), or (b) you build outside $GOPATH with no go.mod file in $PWD.

[bzz]

CI is going to be fixed on master in separate PR #221.
Scope in this PR's description was updated to include last module-related test failure instead.

[bzz]

Needs to be rebased on master, after #222 lands:

• resolve conflicts
• drop last 2 commits
• bump versions as Go 1.12.4 and Go 1.11.9 were just released

[bzz]

As discussed, consensus is that go test all failures in third-party dependencies might happen but do not block this work as soon as our tests pass.

Looking into code generator test fail with \w oniguruma:

$ go test -tags 'oniguruma' ./internal/code-generator/... -run Test_GeneratorTestSuite -testify.m TestGenerationFiles

[bzz]

CI fails due to #225 that should be investigated and fixed in a separate profile.

This is ready to be merged as soon as /v2 is added!

[bzz]

This is ready to be merged now - v2 was added in 6a5f37e

CI will be taken care of under #225 before v2.0.0 release