Add support for Package provides

Add "provides" field in packages (which affect both runtime and buildtime deps).
It replaces all the occurences in the deptree before solving, actually
allowing to swap packages and provide virtuals. Along with a mechanism
for package rename #25.

tests/fixtures/provides/c/build.yaml

@@ -0,0 +1,7 @@
+image: "alpine"
+prelude:
+  - echo foo > /test
+  - echo bar > /test2
+steps:
+  - echo c > /c
+  - echo c > /cd

tests/fixtures/provides/c/definition.yaml

@@ -0,0 +1,7 @@
+category: "test"
+name: "c"
+version: "1.0"
+provides:
+- category: "bar"
+  name: "foo"
+  version: "1.0"

tests/fixtures/provides/d/build.yaml

@@ -0,0 +1,11 @@
+prelude:
+  - echo foo > /test
+  - echo bar > /test2
+steps:
+  - echo s > /d
+  - echo dd > /dd
+requires:
+- category: "bar"
+  name: "foo"
+  version: "1.0"
+

tests/fixtures/provides/d/definition.yaml

@@ -0,0 +1,3 @@
+category: "test"
+name: "d"
+version: "1.0"

tests/fixtures/provides/virtual/foo/definition.yaml

@@ -0,0 +1,3 @@
+name: "foo"
+version: "1.0"
+category: "bar"