Allows the user to override $GO111MODULE environment variable through
ale options. This gives control over the default behavior of Go module
resolution.
Golang documentation:
https://github.com/golang/go/wiki/Modules#how-to-use-modules
Add `ale#Go#EnvString()` function to make it easy to add similar Go
environment variables in the future.
Use the new `EnvString` function in all available Go tools callbacks
& update tests
Also add test of linter command callback for `gofmt`
I see no reason to do this? It is just setting the environment to what
it already is?
It was originally added in #297, but that entire PR is not a great idea
in the first place; that PR (together with #270) tried to make the Go do
non-standard and non-supported stuff like compiling packages outside of
GOPATH.
That's not something that works well (I tried), so was eventually
removed in #465, but these "go env" calls remained, for no reason in
particular, as far as I can think of.
This will improve on #1834; you will now no longer get a confusing error
(but still won't get a meaningful error; need to think how to do that).
