[WIP] get: try to remove destination directory before renaming download

[schomatis]

If the package.json file is missing from an installed package (not very likely) fetch will fail while trying to rename to an existing directory.

rm ~/go/src/gx/ipfs/QmdbxjQWogRCHRaxhhGnYdT1oQJzL9GdqSKzCdqWr85AP2/pubsub/package.json
gx install
# ERROR: [97 / 97 ] parallel fetch: failed to fetch package: QmdbxjQWogRCHRaxhhGnYdT1oQJzL9GdqSKzCdqWr85AP2: rename go/src/gx/ipfs/QmdbxjQWogRCHRaxhhGnYdT1oQJzL9GdqSKzCdqWr85AP2.part go/src/gx/ipfs/QmdbxjQWogRCHRaxhhGnYdT1oQJzL9GdqSKzCdqWr85AP2: file exists
# ERROR: install deps: failed to fetch dependencies

Should a test be added?

[djdv]

@schomatis
I created a similar patch to this #163
It moves the check up so it only tries to remove when proven to exist and also removes an uneeded dependency.

Sorry for the overlap, I came across this independently when I got frustrated with an ambiguous "Access is denied" error. I didn't see this pull request when I started.

I can instigating failure by doing this inside of the go-ipfs repo

gx install --local
git clean -fdx
gx install --local

git clean -fdx was not removing any packages with tests in them which caused the problem when trying to move the temporary to the target.

[whyrusleeping]

@schomatis sorry I didnt see this one either. I'm merging @djdv's PR as both of these fixes work, but he points out that go's os package now properly does the rename, we no longer need the gorename package.

thank you for the PR!

[schomatis]

@djdv No problem, I'm closing this PR as I like your solution more, it employs the standard library and handles the existence of the destination in a logically coherent place: next to the original check of FindPackageInDir, even if the Shell().Get() fails (for other reasons) it makes sense that the destination should be removed early in the function.

[schomatis]

Oh, @whyrusleeping already took care of it :)