Add support for Go 1.16

Go 1.16 now rejects empty go.mod files (c.f. https://github.com/golang/go/commit/d4986e0e1d6e03e1b92e905ca5e01b4c223fbeb3). This means that golang hooks cannot be installed anymore:

$ pre-commit install-hooks
[INFO] Initializing environment for local.
[INFO] Installing environment for local.
[INFO] Once installed this environment will be reused.
[INFO] This may take a few minutes...
An unexpected error has occurred: CalledProcessError: command: ('/usr/local/bin/go', 'get', './...')
return code: 1
expected return code: 0
stdout: (none)
stderr:
    go: no module declaration in go.mod. To specify the module path:
    	go mod edit -module=example.com/mod
    
Check the log at /Users/rdraoui/.cache/pre-commit/pre-commit.log

I could get it to work by changing pre_commit/resources/empty_template_go.mod from an empty file to:

module example/mod

But I’m not all that familiar with how modules work in different Go versions, so maybe making that change has some other implications that I don’t know about. This Go Wiki entry seems to suggest that empty go.mod files are legal, but maybe it’s outdated… (c.f: https://github.com/pre-commit/pre-commit/pull/1428#pullrequestreview-404547009)

The fix is compatible with Go 1.15.

I tried validating the fix by running the test suite (pytest tests -k golang), but I can’t get it to pass with Go 1.15 and current master branch either.

[asottile]

this is the minimal patch to fix repo: local for golang (as far as I can tell, all other usages of go work already):

$ git diff
diff --git a/pre_commit/resources/empty_template_go.mod b/pre_commit/resources/empty_template_go.mod
index e69de29..fc0ab8c 100644
--- a/pre_commit/resources/empty_template_go.mod
+++ b/pre_commit/resources/empty_template_go.mod
@@ -0,0 +1,3 @@
+go 1.11
+
+module pre-commit-dummy-empty-module

the testsuite however uses classical gopath-based modules which apparently are outright broken by go 1.16 unless GO111MODULE=auto is set (which is outside the control of pre-commit)

[asottile]

hmm yeah it might just be that we need to copy the repo remote along here: https://github.com/pre-commit/pre-commit/blob/f3de91c2bb2d5dfefa2e9ad737cfa516964fe85e/pre_commit/languages/golang.py#L72-L73