Cache imported packages

[charlievieth]

This commit adds a types.Importer that caches packages. The uses the filepath of the object/archive file associated with a package as the key so that vendoring is supported (it also makes the cache relatively simple). The timestamp of the object/archive file is used to determine when a package should be re-imported.

Note: There is already an open PR to add caching (#51), but being that I've already written this and that it's a little more portable (since it implements a types.ImporterFrom that may be used by gbimporter instead of modifying gbimporter itself) I'm creating the PR. Happy to close, if the solution proposed in #51 is preferable.

[charlievieth]

Tests pass on go1.10.3 and go1.11.0 on Darwin and Linux. The tests fail on the latest development version of Go:

go version devel +4545fea4fc Sat Sep 8 23:59:26 2018 +0000 linux/amd64

The reason for the failure is that func MapRange() *reflect.MapIter is now included in the result set, which is correct. The MapRange method was added with commit golang/go@ede5958 which exists on master, but is not in the go1.11.x or go1.10.x release branches.

If MapRange is included in the next release of Go the tests will need to be updated.

[charlievieth]

bump

[stamblerre]

There is already a branch that adds caching, and I think it's pretty similar to what you have here. I can investigate merging the branch into master, but I don't think we will accept this since it's too similar. Sorry about that!