Rework of bucket path handling in resultstore.go

[maxfisher-g]

This PR makes changes to the ResultStore type to simplify bucket URL and path processing.

The change is needed due to google/go-cloud#3294 which means special handling of file bucket URLs is required. Also fixes #811.

• workaround for fileblob: os.Rename does not work between different mount points google/go-cloud#3294 by adding ?no_tmp_files=true to file bucket URLs
• use URL object to parse and store bucket path on construction
• remove extra parsing of bucket URL in cmd/analyze/main.go
• rename the generatePath function to generateKey which takes in the filename and prefixes it with the package info if the constructPath option is used
• remove and inline MakeFilename function which was only used in two places and with different options
• add internal function for opening a bucket, which exposes it for unit tests
• add unit tests for opening and writing to a file bucket (todo: test other types of bucket)

More unit tests should be added to test the bucket type used by the worker. This can be done in another PR before the next release.