Make builds able to depend on external projects (#291)

* Make builds able to depend on external projects

Builds are now able to load external projects and depend on them
as if they were local submodules.

`import $file.external.path.build`

* Disambiguate "dest" for foreign modules.

* Calling modules loaded from external directories "Foreign" to avoid
conflicting with the already existing concept of "ExternalModule".
* Amended the way `dest` is computed for foreign modules
* Added tests to check that the source paths and dest are as expected
* Added a test to show that local modules do not conflict with foreign
modules when they are named the same

* WIP windows build fail

* Added bootstrapping step in CYGWIN CI job

* * Revert externalOutPath deletion
* Add documentation for foreign-modules

* reverting appveyor changes

* Disabling Foreign modules tests against Java9

See https://github.com/lihaoyi/mill/issues/302

2 files changed, 28 insertions, 0 deletions

diff --git a/main/test/resources/examples/foreign/conflict/build.sc b/main/test/resources/examples/foreign/conflict/build.sc
new file mode 100644
index 00000000..d6c08b81
--- /dev/null
+++ b/main/test/resources/examples/foreign/conflict/build.sc
@@ -0,0 +1,24 @@
+import $file.inner.{build => innerBuild}
+import mill._
+import ammonite.ops._
+
+// In this build, we have a local module targeting
+// the 'inner sub-directory, and an imported foreign
+// module in that same directory. Their sourcePaths
+// should be the same, but their dest paths should
+// be different to avoid both modules over-writing
+// each other's caches .
+
+def checkPaths : T[Unit] = T {
+  if (innerBuild.millSourcePath != inner.millSourcePath)
+    throw new Exception("Source paths should be the same")
+}
+
+def checkDests : T[Unit] = T {
+  if (innerBuild.selfDest == inner.selfDest)
+    throw new Exception("Dest paths should be different")
+}
+
+object inner extends mill.Module {
+  def selfDest = T { T.ctx().dest / up / up }
+}
diff --git a/main/test/resources/examples/foreign/conflict/inner/build.sc b/main/test/resources/examples/foreign/conflict/inner/build.sc
new file mode 100644
index 00000000..729f4f3d
--- /dev/null
+++ b/main/test/resources/examples/foreign/conflict/inner/build.sc
@@ -0,0 +1,4 @@
+import mill._
+import ammonite.ops._
+
+def selfDest = T { T.ctx().dest / up / up }