Build beats outside GOPATH

[kvch]

What does this PR do?

This PR adds support for building Beats outside GOPATH by separating repo information collection depending on the root path of the project.

Why is it important?

This lets users compile and package Beats if they have checked out the code to a non-GOPATH folder.

Checklist

• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have made the corresponding change to the default configuration files
• I have added tests that prove my fix is effective or that my feature works

[kvch]

@andrewkroh Could you please take a look at this PR?

[andrewkroh]

I think a few changes are needed. Here's my mental model of the algorithm:

1. Traverse upward looking for the projects go.mod file. This is the RootDir.
2. Get the SubDir value by using filepath.Rel(RootDir, CWD()).
3. Read the go.mod file to get the RootImportPath.
4. Get the ImportPath by filepath.ToSlash(filepath.Join(RootImportPath, SubDir)).

You'll definitely want to go through the code and check the usages of RootImportPath and ImportPath. Just to make sure that no assumptions about their values that will break things if the go.mod module path differs from github.com/elastic/beats (or in general differs from the earlier canonical go import paths).

And the check for IsElasticBeats() should probably change to a HasPrefix type of check in case the go.mod module path grows to include versions (e.g. github.com/elastic/beats/v8).

[andrewkroh]

I think this looks good now 👍 . I do have some suggestions for naming and comments that will hopefully help others to understand and maintain this.