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`
* Added filename keys to gobuild and gometalinter
* Removed skipping files not in current package
* Removed `--include` for gometalinter
* Fixed the tests
The real fix was not using absolute paths anymore (so not expanding with the `:p` option). The regex was correct and should at least include the `^` character to make sure the string starts with the given path/filename and not references the path/filename in some error description.
