Although the accepted answer is still correct about needing to match directories with package names, you really need to migrate to using Go modules instead of using GOPATH. New users who encounter this problem may be confused about the mentions of using GOPATH (as was I), which are now outdated. So, I will try to clear up this issue and provide guidance associated with preventing this issue when using Go modules.
If you're already familiar with Go modules and are experiencing this issue, skip down to my more specific sections below that cover some of the Go conventions that are easy to overlook or forget.
This guide teaches about Go modules: https://golang.org/doc/code.html
Project organization with Go modules
Once you migrate to Go modules, as mentioned in that article, organize the project code as described:
A repository contains one or more modules. A module is a collection of related Go packages that are released together. A Go repository typically contains only one module, located at the root of the repository. A file named go.mod there declares the module path: the import path prefix for all packages within the module. The module contains the packages in the directory containing its go.mod file as well as subdirectories of that directory, up to the next subdirectory containing another go.mod file (if any).
Each module's path not only serves as an import path prefix for its packages, but also indicates where the go command should look to download it. For example, in order to download the module golang.org/x/tools, the go command would consult the repository indicated by https://golang.org/x/tools (described more here).
An import path is a string used to import a package. A package's import path is its module path joined with its subdirectory within the module. For example, the module github.com/google/go-cmp contains a package in the directory cmp/. That package's import path is github.com/google/go-cmp/cmp. Packages in the standard library do not have a module path prefix.
You can initialize your module like this:
$ go mod init github.com/mitchell/foo-app
Your code doesn't need to be located on github.com for it to build. However, it's a best practice to structure your modules as if they will eventually be published.
Understanding what happens when trying to get a package
There's a great article here that talks about what happens when you try to get a package or module: https://medium.com/rungo/anatomy-of-modules-in-go-c8274d215c16 It discusses where the package is stored and will help you understand why you might be getting this error if you're already using Go modules.
Ensure the imported function has been exported
Note that if you're having trouble accessing a function from another file, you need to ensure that you've exported your function. As described in the first link I provided, a function must begin with an upper-case letter to be exported and made available for importing into other packages.
Names of directories
Another critical detail (as was mentioned in the accepted answer) is that names of directories are what define the names of your packages. (Your package names need to match their directory names.) You can see examples of this here: https://medium.com/rungo/everything-you-need-to-know-about-packages-in-go-b8bac62b74cc
With that said, the file containing your main method (i.e., the entry point of your application) is sort of exempt from this requirement.
As an example, I had problems with my imports when using a structure like this:
/my-app
+-- go.mod
+-- /src
+-- main.go
+-- /utils
+-- utils.go
I was unable to import the code in utils into my main package.
However, once I put main.go into its own subdirectory, as shown below, my imports worked just fine:
/my-app
+-- go.mod
+-- /src
+-- /app
| +-- main.go
+-- /utils
+-- utils.go
In that example, my go.mod file looks like this:
module git.mydomain.com/path/to/repo/my-app
go 1.14
When I saved main.go after adding a reference to utils.MyFunction(), my IDE automatically pulled in the reference to my package like this:
import "git.mydomain.com/path/to/repo/my-app/src/my-app"
(I'm using VS Code with the Golang extension.)
Notice that the import path included the subdirectory to the package.
Dealing with a private repo
If the code is part of a private repo, you need to run a git command to enable access. Otherwise, you can encounter other errors This article mentions how to do that for private Github, BitBucket, and GitLab repos: https://medium.com/cloud-native-the-gathering/go-modules-with-private-git-repositories-dfe795068db4 This issue is also discussed here: What's the proper way to "go get" a private repository?