test(pkg): file-depends (#9035)

* test(pkg): file-depends

We test the file-depends field of the .config file of an opam package
and demonstrate that currently it doesn't work as expected.

Signed-off-by: Ali Caglayan <alizter@gmail.com>

test/blackbox-tests/test-cases/pkg/file-depends.t

@@ -0,0 +1,60 @@
+Here we test the file-depends field in pkg1.config. When a package has been
+installed, the .config file can also be included. This can have a file-depends
+field which is a list of external files, together with their checksums, that
+the package depends on. We make sure that such a package really does depend on
+the files found in files-depend. 
+
+  $ . ./helpers.sh
+  $ make_lockdir
+
+  $ foo=$PWD/foo
+  > cat > dune.lock/file-depends.pkg <<EOF
+  > (build
+  >  (system "\| echo Building file-depends
+  >          "\| cat > file-depends.config <<EOF
+  >          "\| opam-version: "2.0"
+  >          "\| file-depends: [ "$foo" "md5=00000000000000000000000000000000" ]
+  >          "\| EOF
+  >  ))
+  > EOF
+
+Word of warning: the opam libraries will quietly discard the file-depends field if the
+checksum is not parsable.
+
+Now we make a package depending on file-depends.
+
+  $ cat > dune.lock/dep.pkg <<EOF
+  > (deps file-depends)
+  > (build
+  >  (system "echo Building dep"))
+  > EOF
+
+  $ cat > foo <<EOF
+  > Hello
+  > EOF
+
+Building dep should show both of them as being built.
+
+  $ build_pkg dep
+  Building file-depends
+  Building dep
+
+Building again causes no rebuild as expected.
+
+  $ build_pkg dep
+
+Changing foo should cause foo to be rebuilt.
+
+  $ cat > foo <<EOF
+  > World
+  > EOF
+
+CR-someday alizter: This is broken, no rebuild is done.
+  $ build_pkg dep
+
+Removing foo should cause an error due to the missing file.
+
+  $ rm foo
+
+CR-someday alizter: This is broken, no rebuild is done.
+  $ build_pkg dep