x/text: add go modules support (based on ssoroka pr #9)

[kckrinke]

Updated version of the original pull request #9.

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Patch Set 1: Code-Review+2

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[kckrinke]

Please note that the github repository for this PR is being archived and deleted at some point. The new home for things is over at: https://github.com/go-corelibs/x-text

If this PR needs to be resubmitted or otherwise modified, just let me know what to do.