Improve handling of paths containing repeated separators (fix #1946).

[jpentland]

What does this do / why do we need it?

Issue: #1946

Host system GOPATH may contain repeated separators or other artifacts which can cause an error
when not needed. E.g:

GOPATH:

/home/user//.gopath
/home/user/./.gopath
/home/user/code/../.gopath

Error:

/home/user/.gopath/src/github.com/project/repo is not within a known GOPATH/src

The function filepath.Clean() is used in order to simplify filepaths and remove the artifacts.

What should your reviewer look out for in this PR?

In order to fix the issue only the changes to internal/fs/fs.go are required. However context.go is also
updated to further clean the GOPATH as it gets returned from TestDetectGOPATH.

Test cases were added in context_test.go and internal/fs/fs_test.go, which currently only cover repeated
slashes, since this was what the bugfix is targeting.

There is also one small fix to the test case TestDetectGOPATH where the two string values were reversed in the error statement.

Do you need help or clarification on anything?

I'm not sure if the test case changes should live in separate commits to the actual code fix.

Which issue(s) does this PR fix?

fixes #1649

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[jpentland]

I signed it!

[googlebot]

CLAs look good, thanks!

[googlebot]

CLAs look good, thanks!

[sdboyer]

Looks like it should be fine!

Changes to this logic has historically given us some nasty bites, but it really seems like a normalizing function like filepath.Clean() should be safe.

So, as long as the tests pass, best thing to do is just give it a try.