Issue Description:

New in Swift 5.4, certain sequences of commands seem to scramble the build plan cache such that it ends up building far flung executables, dynamic libraries, and explicitly static libraries even though they are unreachable from the root package.

Steps to reproduce:

1. Create a new executable package and declare a dependency on one product in a package which also has other products that generate binaries (such as an executable, a .dylib or a .a). The SDGSwiftConfiguration product from https://github.com/SDGGiesbrecht/SDGSwift at 6.0.1 will suffice to demonstrate.

2. Build the package

swift build

As expected, it will only build the two necessary modules (SDGSwiftConfiguration and the executable itself).

3. Run the executable

swift run the‐name‐of‐the‐executable

As expected, it will launch the executable and print “Hello, world!” or whatever.

4. Build the package again

swift build

Now it will incorrectly build every binary product in any manifest in the dependency graph, even the ones unreachable from the root package (such as SwiftSyntax.a in this case).

Notably, the problem does not occur if step 3 is skipped.