all: move all examples to external test packages

[josharian]

This is an extraction from #11254.

Having the package prefix in examples is clearer than omitting it. Since examples must not make use of unexported functionality anyway, it is always ok to have them in an external test package. Let's move all examples to external test packages.

If Go 1 compatibility didn't prevent it, I'd suggest that we modify the testing package to enforce this.

[josharian]

Filed golang/lint#135 to add a lint check for this.

[adg]

I could only find one such set of examples, in package mime. CL incoming.

My command line for finding them is:

grep '^package' $(git grep -l '^func Example') | grep -v '_test$'

[gopherbot]

CL https://golang.org/cl/13779 mentions this issue.