We already import and use build package here, so there's no harm in
reuse. The code is both shorter and more readable.

Look for import comments following the package declaration. The
implementation of findImportComment() very closely follows go/build's
findImportComment().

Tests and validation against importRoot to come in a later commit.

Added a check and an error type for importRoot to be a prefix of
canonical import path, tests for the new code paths.

Fixed one unrelated bug along the way:
    Sprintf("%q", strings.Join(strs, "\", \""))
doesn't work as intended, since the %q verb will escape the inner
doublequotes.