Rewrite LSIF upload command

Created by: efritz

This PR introduces behavior that will only be available in Sourcegraph 3.16 (via https://github.com/sourcegraph/sourcegraph/pull/10456).

Behavioral changes:

• Moved reading the indexer name from the precise-code-intel-api-server to here so we don't have to write to disk in the upload path (before re-proxying to the bundle manager). Users can still specify the indexer name via a flag.
• Added -json flag to output a machine-readable to aid in integration tests. This will become more popular as we add additional precise-code-intel actions that will help indexing monorepos.
• Removed -get-curl for the lsif upload action as we can't guarantee a bounded number of HTTP requests anymore.
• Added support for chunking the LSIF index into parts and making several requests when the file size is too large. This threshold is default 100Mb, but is configurable with a flag for environments that are more or less constrained.

Code changes:

• Introduced internal/codeintel package
• Added tests

Build changes:

• New tests require they be run inside a git root, so the shallow_clone option has been removed from the appveyor config.

Notes to reviewers:

• Don't try to read the diff for lsif_upload.go, just read the new code.