Several changes in build-script-helper.py to support building indexstore-db in a unified build

[ahoppen]

This contains the analogous changes from swiftlang/sourcekit-lsp#597 as well as the following additional ones:

• Add SWIFTCI_USE_LOCAL_DEPS=1 environment variable when invoking SwiftPM
  • So far, this has not been relevant because indexstore-db didn’t have any dependencies. Once we build indexstore-db in a unified SwiftPM the swift build invocation will also resolve Package.swift for the other packages in the unified build, which need this environment variable.

[benlangmuir]

This contains the analogous changes from apple/sourcekit-lsp#597 as well as the following additional ones:

• Build all libraries as static libraries instead of auto

  • Otherwise, we can’t explicitly build these libraries by specifying e.g. --product IndexStoreDB. But we need to be able to request explicit builds once we build indexstore-db in a unified SwiftPM build.
  • I’m open to debate whether these should be .dynamic instead of .static but static libraries appear to work at the moment.

Since we don't "install" this product anywhere currently, how about we just use the test target instead? Then we can keep the main products 'auto'.

--product IndexStoreDBPackageTests