Goto Definition and symbols containing a period

Language.sublime-syntax

%YAML 1.2
---
# See http://www.sublimetext.com/docs/3/syntax.html
file_extensions:
  - lang
scope: source.lang
contexts:
  main:
    - match: '//'
      scope: punctuation.definition.comment.lang
      push: line_comment

    - match: '([\w.]+):{'
      captures:
        1: entity.name.function

    - match: '([\w.]+)(\[)'
      captures:
        1: support.function.call

  line_comment:
    - meta_scope: comment.line.lang
    - match: $
      pop: true

Language.tmPreferences

<?xml version="1.0" encoding="UTF-8"?>
<plist version="1.0">
<dict>
    <key>scope</key>
    <string>source.lang entity.name.function</string>
    <key>settings</key>
    <dict>
        <key>showInSymbolList</key>
        <integer>1</integer>
        <key>showInIndexedSymbolList</key>
        <integer>1</integer>
    </dict>
</dict>
</plist>

sample.lang

// Function 1
function.name1:{}

// Function 2
function.name2:{}

// Function 3
function.name3:{}

// Function 4
function.name4:{}

// Function 4
function.name5:{}

.namespace.function:{}

function.name1[]
function.name2[]
function.name3[]
function.name4[]
function.name5[]

.namespace.function[]

As defined, the goto definition code will not work with this syntax unless you select the entirety of the symbol before invoking it. This is because the method symbol_at_point in Packages\Default\symbol.py is hard coded to assume various word characters (instead of pulling them from a setting or some such).