expand user-supplied args

[HunterLarco]

Changes the rollup rule to expand location expressions in user-supplied args.

For example, as if this PR one could:

rollup(
  name = "example",
  args = ["--configCustomField", "$(location :foo)"],
  srcs = [":foo"],
)

js_library(
  name = "foo",
  srcs = ["foo.js"],
)

Typical usages of rollup will not need this feature and will remain unaffected, however, rollup does support custom command line arguments and in those cases it can be useful to pass a build-time data file. Location expansion allows users to express this without hardcoding bazel file structures (see the above example).

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Changes are visible to end-users: yes

• Searched for relevant documentation and updated as needed: yes
• Breaking change (forces users to change their own code or config): no
• Suggested release notes appear below: no

Test plan

I'm happy to add new tests, just let me know. The existing tests don't appear to check the contents of the bundle which would be necessary to fully test this feature. That said it's relatively straightforward so I could see this not requiring new tests.

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Test

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Buildifier

[jbedard]

I think arg (not args) should be passed to expand_location?

[jbedard]

See the write_source_files tests that are asserting the output is what's expected, I think you'd do the same to test this.