Add support for ignoring imports from files with specific build constraints

[blitzrk]

Add --skip-tag flag to choose to ignore imports from files with any of the given build tags. In many cases, adding a --skip-tag ignore and --skip-tag appengine could help limit the number of unneeded dependencies discovered with --update.

PR incoming...

[yamamoto-febc]

Hi @blitzrk,

I have a few questions about --skip-tag.
If --skip-tag appengine is specified, what happens to the file with the following build tags?

// +build linux appengine

If this file will skipped, you will not be able to build on Linux.

If the target tag is ignore, I think it will work.
But in other cases, build may become impossible.

What do you think?