d7_file plugin should receive source_base_path configuration option

Problem/Motivation

As seen in #2547125: D7 file migration should allow selecting public/private/all files, it's necessary for the d7_file source plugin to pull information about the file system out of the destination plugin's configuration. This is, in @mikeryan's words, horrifying.

Proposed Resolution

Pass source_base_path in the d7_file source's configuration.

Remaining Tasks

SOP. Patch, test, review, commit.

API Changes

None.

Data Model Changes

Not sure if this counts, but the d7_file plugin's configuration schema may include new configuration options.