Move all requirements files into "tools" directory

These new `.in` and `.txt` are making the repository look very
cluttered. Move them to a separate `tools` directory to make it look a
bit cleaner.

MANIFEST.in

@@ -32,12 +32,12 @@ include .pylintrc
 recursive-include buildstream/_protos *.proto
 
 # Requirements files
-include requirements.in
-include requirements.txt
-include dev-requirements.in
-include dev-requirements.txt
-include plugin-requirements.in
-include plugin-requirements.txt
+include tools/requirements.in
+include tools/requirements.txt
+include tools/dev-requirements.in
+include tools/dev-requirements.txt
+include tools/plugin-requirements.in
+include tools/plugin-requirements.txt
 
 # Versioneer
 include versioneer.py

setup.py

@@ -270,10 +270,10 @@ def get_cmdclass():
 #####################################################
 #               Gather requirements                 #
 #####################################################
-with open('dev-requirements.in') as dev_reqs:
+with open('tools/dev-requirements.in') as dev_reqs:
     dev_requires = dev_reqs.read().splitlines()
 
-with open('requirements.in') as install_reqs:
+with open('tools/requirements.in') as install_reqs:
     install_requires = install_reqs.read().splitlines()
 
 #####################################################

tox.ini

@@ -5,9 +5,9 @@ skip_missing_interpreters = true
 [testenv]
 commands = pytest {posargs}
 deps =
-    -rrequirements.txt
-    -rdev-requirements.txt
-    -rplugin-requirements.txt
+    -rtools/requirements.txt
+    -rtools/dev-requirements.txt
+    -rtools/plugin-requirements.txt
 passenv =
     GI_TYPELIB_PATH
     INTEGRATION_CACHE