v40

When downloading a dependency, create the base directory if needed.

v39: Record / Update only the major go version

After some issue discussion (heroku/heroku-buildpack-go/100 and another
I can't find atm) I generally think that we should really only care
about the major version of Go and let downstream systems care about
selecting the minor version (the user or build systems). There are
probably exceptions to that, hence I also stopped auto-updating the go
version on save, allowing people to specify whatever they want if they
manually want to manage the file.

v38: Replace `go get`, further fix up error handling/reporting

Fixes another issue reported in #186 where an entire package is missing.

Don't bother restoring/downloading if already done.

v37: Change up how download/restore works a little

* Try to load the package after downloading/restoring. Previously
  that was done too early in the process.
* make previous verbose output debug output
* report a typed error instead of a string from listPackage so it can
  be asserted to provide a nicer error.
* Catch go get errors that say there are no go files found. See code
  comment as to why.
* do *all* downloading during download phase.

v36: wrong variable name

In v35 I renamed this variable to differentiate, but missed one
location.

This code isn't being tested well then. Will add a test after releasing
v36 to fix this issue.

Fixes #358

v35

Merge branch 'performance_regression'

v34

Merge branch 'fixes'

v33: Replace `go list`

Main motivation is so that we can attempt to pick up packages across
architectures and to have more control over the list of packages.

I consider this the first in several structural changes.

Most additions are inside of list.go.

For each run of listPackage a depScanner is made to track already seen
packages (inspired by Scanner). That list is not cached between
executions.

The current implementation of listPackage attempts to resolve vendor/ a
bit better than we used to, but other changes need to be implemented
elsewhere in godep before that can be fully realized.

All *existing* tests pass, but this doesn't mean it actually works
*better*.

A test was added to show that we fix #271.

v32: EvalSymlinks in Contains() check

git's root command (maybe others vcs as well) resolve symlinks, so try
that too.

PS: In git's case we can possible use rev-parse --show-cdup + extra
logic to find the root of the repo instead. Need to figure out what
other vcs root commands do as well.

Fixes #339

v31: In restore errors, mention which package had the problem.

This improves the situation when the user runs `godep restore`
and sees a few instances of "restore, during download dep" errors.
Previously, it was not mentioned which packages failed to download.
Turning on -v verbose mode was not a good solution since it prints
too much other information for successfully processed packages.