Fix some links in proposals (#1090)

proposals/p0107.md

@@ -268,7 +268,7 @@ Advantages:
 Disadvantages:
 
 -   We are likely to want a more fine-grained, file-level approach proposed by
-    [name lookup](name_lookup.md).
+    [name lookup](/docs/design/name_lookup.md).
 -   Allows package owners to name their packages things that they rarely type,
     but that importers end up typing frequently.
     -   The existence of a short `package` keyword shifts the balance for long

proposals/p0339.md

@@ -221,8 +221,8 @@ something like `let mut` in Rust. However, this is not assumed by this proposal:
     appropriately
     [focuses on encouraging appropriate usage of features rather than restricting misuse](/docs/project/goals.md#code-that-is-easy-to-read-understand-and-write)).
 -   Lower verbosity syntax for variables is more
-    [consistent with C++](c-as-baseline), even if constants are made less
-    verbose by way of `let`.
+    [consistent with C++](p0285.md#c-as-baseline), even if constants are made
+    less verbose by way of `let`.
 
 ### Multiple identifiers in one statement
 

proposals/p0722.md

@@ -46,7 +46,7 @@ which laid a foundation.
 
 ## Proposal
 
-The proposal is to update [docs/design/classes.md](docs/design/classes.md) as
+The proposal is to update [docs/design/classes.md](/docs/design/classes.md) as
 described in [this PR](https://github.com/carbon-language/carbon-lang/pull/701).
 
 ## Rationale based on Carbon's goals

proposals/p0731.md

@@ -191,8 +191,9 @@ One benefit is that it allows an interface to evolve by adding an associated
 type, without having to then modify all implementations of that interface.
 
 One concern is this might be a little more complicated in the presence of method
-overloads with [default implementations](interface-defaults), since it might not
-be clear how they should match up, as in this example:
+overloads with
+[default implementations](/docs/design/generics/details.md#interface-defaults),
+since it might not be clear how they should match up, as in this example:
 
 ```
 interface Has2OverloadsWithDefaults {

proposals/p0777.md

@@ -56,7 +56,7 @@ This is a follow up to these previous proposals defining classes:
 
 ## Proposal
 
-The proposal is to update [docs/design/classes.md](docs/design/classes.md) as
+The proposal is to update [docs/design/classes.md](/docs/design/classes.md) as
 described in [this PR](https://github.com/carbon-language/carbon-lang/pull/777).
 
 ## Rationale based on Carbon's goals

proposals/p0998.md

@@ -45,7 +45,7 @@ Proposal is to use interfaces as the single open extension mechanism.
 ## Details
 
 Details are in the added principle doc:
-[docs/project/principles/static_open_extension.md](docs/project/principles/static_open_extension.md).
+[docs/project/principles/static_open_extension.md](/docs/project/principles/static_open_extension.md).
 
 ## Rationale based on Carbon's goals