[acceptance-tests] Move git "submodules" to acceptance-tests/external instead of next to Mono repo

A few people told me it's surprising that acceptance-tests checks out repos into <mono repo>/../
and it can cause unwanted changes when a repo with the same name already exists there.

While this structure is used in several internal repos I agree for Mono it's better to keep
everything inside the repo itself.

acceptance-tests/Makefile.am

@@ -1,7 +1,7 @@
-TOP=..
-ROSLYN_PATH=$(TOP)/../roslyn
-CORECLR_PATH=$(TOP)/../coreclr
-MSTESTSUITE_PATH=$(TOP)/../ms-test-suite
+ACCEPTANCE_TESTS_PATH=external
+ROSLYN_PATH=$(ACCEPTANCE_TESTS_PATH)/roslyn
+CORECLR_PATH=$(ACCEPTANCE_TESTS_PATH)/coreclr
+MSTESTSUITE_PATH=$(ACCEPTANCE_TESTS_PATH)/ms-test-suite
 
 include versions.mk
 

acceptance-tests/external/.gitignore

@@ -0,0 +1,2 @@
+*
+!.gitignore

acceptance-tests/versions.mk

@@ -118,7 +118,7 @@ __bump-branch-%:
 	@if [ "$(COMMIT)" = "1" ]; then echo "[acceptance-tests] Bump $* to switch to $(BRANCH) $(REMOTE BRANCH)." | git commit -F - $(CONFIG); fi
 
 __bump-current-version-%:
-	REV=$(shell cd $(TOP)/../$* && git log -1 --pretty=format:%H); \
+	REV=$(shell cd $(ACCEPTANCE_TESTS_PATH)/$* && git log -1 --pretty=format:%H); \
 	python versions.py set-rev $* $$REV; \
 	if [ "$(COMMIT)" = "1" ]; then echo "[acceptance-tests] Bump $* to pick up $$REV:" | git commit -F - $(CONFIG); fi