dl/internal/version, dl/gotip: merge gotip into internal/version

[zikaeroh]

dl/internal/version and dl/gotip contain much of the same code. Fixes for issues like #36976 need to be applied to both, even though their underlying operation is nearly identical (including duplicated helpers and tests). internal/version could back gotip instead.

Splitting out from #36976 (comment).

I have a CL chain I'm planning to submit to clean this up (and fix #36976).

[gopherbot]

Change https://golang.org/cl/217764 mentions this issue: internal/version, gotip: move gotip into internal/version

[dmitshur]

/cc @dmitshur @toothrot @cagedmantis

[zikaeroh]

Any chance my pair of CLs can be reviewed? Not entirely certain who reviews for the dl repo, but I'm hopeful now that 1.14 is out. 🙂

[gopherbot]

Change https://golang.org/cl/301909 mentions this issue: gotip: fold into internal/version